Refactor the current function call deduction logic to make it reusable.
Call into it from `impl` deduction. Also build a generic region for the
definition portion of a generic `impl` and substitute into it before
accessing the witness in a specific `impl`.
This is enough to get simple uses of generic `impl`s to work. The main
blocker for more complex cases is that we have very little support for
non-trivial deduction, so while we can deduce `forall [T:! type] T as
I`, we can't deduce `forall [T:! type] C as I(T)` yet.
Add these interfaces to the core library. For now, they're two separate
interfaces because we don't yet support one interface extending another.
This collapses a lot of the layering in check: for example, the call
building logic depends on implicit conversions, conversions now depend
on the overloaded operator machinery, and that machinery depends on
building calls.
In passing, improve the diagnostics for failing to find a name required
from the prelude. Also convert all the transitively-called code from
`NodeId` to `LocId` given the latter is what the conversion machinery
has available.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Richard Smith
