## Summary
The existing signature for `str` calls had various problems, one of
which I noticed while looking at some ecosystem projects (`scrapy`,
added as a project to mypy_primer in this PR).
## Test Plan
- New tests for `str(…)` calls.
- Observed reduction of false positives in ecosystem checks
This updates the `Signature` and `CallBinding` machinery to support
multiple overloads for a callable. This is currently only used for
`KnownFunction`s that we special-case in our type inference code. It
does **_not_** yet update the semantic index builder to handle
`@overload` decorators and construct a multi-signature `Overloads`
instance for real Python functions.
While I was here, I updated many of the `try_call` special cases to use
signatures (possibly overloaded ones now) and `bind_call` to check
parameter lists. We still need some of the mutator methods on
`OverloadBinding` for the special cases where we need to update return
types based on some Rust code.
