Leverage function contract information

[yannham]

Closes #1885.

When a function is annotated with a function contract, use this additional type information to enable completion in the LSP (and also use the domain as the type of the function's argument in the typing environment, even if we are in walk mode). Contract data are easy to fetch during typechecking when available and use them improve the developer experience by bringing in more static information.

I hesitated to updated the documentation about the fact that the typechecker is marginally smarter about function contracts, but I'm not sure yet we want to commit to it fully yet. Honestly I don't expect it to be used a lot in the wild, I mostly did it because it was virtually free (performance-wise and code complexity-wise).

[yannham]

I realized that for some reason, the completion doesn't fire if we don't start to typing a prefix of an existing field, while this doesn't happen for other kind of completion (for type-based completion for example, if you start to type std., you get a list of all available functions - same for a record literal, if you define record = {...} and then later type record., you get a full list - but here we get nothing until we type the first letter of a field). I'll open a separate issue.

[yannham]

In fact it's the good old NLS can't handle some incomplete inputs. If I add an identifier at the end of this line, something like src_val = src.foo. dummy_value, which is then parsed correctly (because right now you can have a space between fields and the . character, for the best or the worst), then you get completion.