Reduce monomorphization in Deserializer implementation

[DaniPopes]

The F generic in MapDe and SeqDe makes it so that every constructed instance has its own type, which creates unnecessary monomorphizations of the underlying Deserialize methods. This is especially noticeable when the type being deserialized is a struct with 100+ fields that automatically derives the Deserialize implementation, as the multiple copies of the visit_map function end up being up to hundreds of KiB, bloating code size and compilation time.

Here's an example output of cargo-bloat on such a configuration:

...
 0.0%   0.1%  64.2KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::Tagged<()> as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#1}>>
 0.0%   0.1%  64.2KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::Tagged<()> as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#2}>>
 0.0%   0.1%  64.2KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::RelativePathBuf as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#2}>>
 0.0%   0.1%  64.2KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::RelativePathBuf as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#0}>>
 0.0%   0.1%  64.2KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::value::Value>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#0}>>
 0.0%   0.1%  64.0KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::Tagged<()> as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#1}>>
 0.0%   0.1%  64.0KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::Tagged<()> as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#2}>>
 0.0%   0.1%  64.0KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::RelativePathBuf as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#2}>>
 0.0%   0.1%  64.0KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::magic::RelativePathBuf as figment::value::magic::Magic>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#0}>>
 0.0%   0.1%  64.0KiB                    foundry_config <<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor as serde::de::Visitor>::visit_map::<figment::value::de::MapDe<figment::value::de::ConfiguredValueDe, <figment::value::value::Value>::deserialize_from<<foundry_config::Config as serde::de::Deserialize>::deserialize::__Visitor, figment::value::de::DefaultInterpreter>::{closure#0}>>

Essentially, each of Tagged<()>, RelativePathBuf, and Value have their own instantiation of MapDe, which duplicates the massive automatically-derived serde visit_map method.

This PR removes this F generic by introducing a separate trait, implemented on the deserializer D, that is just a simple constructor for D.

Another approach would be simply converting the generic F: Fn... to &dyn Fn, but this introduces some runtime overhead, so I've opted for the slightly more verbose approach.