More 'mixed' instances to simplify Expression arithmetic

[matthiasgoergens]

We implement convenient instances to add / multiply / subtract references to expressions.

Nothing changes under the hood, but we move some cloning from the caller to the implementation of arithmetic operations.

[matthiasgoergens]

This used to build on #450, but that's now already merged.

[yczhangsjtu]

I see there are still some clone() in the implementation of the operators. Is it because the original implementation is for Expression<E> instead of &Expression<E>? Is it possible to put the original implementation in impl &Expression<E>, and let other implementations refer to this, instead of the other way round?

[matthiasgoergens]

I see there are still some clone() in the implementation of the operators. Is it because the original implementation is for Expression<E> instead of &Expression<E>?

Yes, that's exactly right.

Is it possible to put the original implementation in impl &Expression<E>, and let other implementations refer to this, instead of the other way round?

Alas, not with the current design. I actually have a prototype PR to enable exactly that. But that's a much larger refactoring, and I still have so many unreviewed clean up PRs left.