Updates to member access (#7697)

Update the rules for member access:
-   Simple member access `a.b`
- If `a` names a scope, performs name lookup and optionally `impl`
lookup.
- Otherwise, `a.b` is shorthand for `a.(typeof(a).b)` and always
performs instance binding.
- Compound member access `a.(m)` does optional `impl` lookup and always
performs instance binding.
- This is a change from only performing instance binding if `m` is an
instance member.
- New operation `a.impl(m)` is introduced. It always performs `impl`
lookup, and nothing else.
- The `BindToType` interface is removed. Only instance binding may be
customized (using the `BindToValue` and `BindToRef` interfaces).

As a result, member access doesn't use whether the right operand is an
instance member anymore. Instead, instance binding is performed whenever
it would be plausible, and a new syntax is used to opt out.

Assisted-by: Gemini via Antigravity

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

docs/design/expressions/member_access.md

@@ -34,12 +34,14 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 ## Overview
 
-> **TODO:** >
+> **TODO:**
 > [#3720: Member binding operators](/proposals/p003720-member-binding-operators.md)
-> introduces an additional "member binding" step, redefines simple member access
-> in terms of compound member access, and defines compound member access in
-> terms of calls to user-implementable interface methods. This document must be
-> updated to reflect those changes.
+> and
+> [#7697: Updates to member access](/proposals/p007697-updates-to-member-access.md)
+> redefine simple member access in terms of compound member access, define
+> instance binding in terms of calls to user-implementable interface methods,
+> and add another member access form `x.impl(M)`. This document must be updated
+> to reflect those changes.
 
 A _qualified name_ is a [word](../lexical_conventions/words.md) that is preceded
 by a period or a rightward arrow. The name is found within a contextually