Implements main typechecking functions as traits

This commit cleans the typechecker code by gathering functions that are
implemented by several Rust types as traits, instead of having a lot of
free-standing variations `walk_foo`, `walk_bar`, etc. This also allows
for some code deduplication for example when walking an array of
walkable element, that can be factored out.

`infer` is a bit of an exception since it relies on `check` and is only
implemented on one type. Still, in the sake of consistency, `infer` gets
its trait as well (including for calling convention - it would be a bit
odd to call `value.check(..)` but `infer(.., value, ..)`).