Disallowing looking up non-constructor functions

[yihozhang]

(assigning @FTRobbin)
Currently, we allow looking up non-constructor functions (i.e., functions whose output is not an EqSort or have a :merge function) in actions. This can cause actions to fail and make the database end up in an intermediate state.

(function f () i64)
(function g () i64 :merge (min old new))
(datatype E)
(function h () E :merge old)
(rule () 
      ((let x (f))))
(rule () 
      ((let x (g))))
(rule () 
      ((let x (h))))
(run 1)

Moreover, in principle, we don't want actions in a rule to observe the database, because such an implicit dependency implies that if the database changes, the rule needs to be fired again on the database. This is why our desugaring phase needs special handling for our semi-naive evaluation to be sound.

A fix to this is that we should disallow looking up values of functions in actions. By doing so, we can also remove the special handling for our semi-naive evaluation. Note that we should still allow global actions to look up functions, so the following program should still be allowed:

(function f () i64)
(let x (f)) ;; typechecks and throws a run-time error

[FTRobbin]

This overrides #228 #197 #142.

This test from #93 would be obsolete:
https://github.com/egraphs-good/egglog/blob/757c52f6dcf8fe5b7b5eefb4bdc872ef70a5433a/tests/semi_naive_set_function.egg

Related code:
Extra desugar that ensures the soundness of the semi-naive evaluation:

egglog/src/ast/desugar.rs

Line 110 in 757c52f

// TODO(yz): we can delete this code once we enforce that all rule bodies cannot read the database (except EqSort).