Move self to the explicit () parameter list (proposal #7016) (#7272)

Implements proposal #7016: `self` moves from the deduced implicit list
(`fn F[self: Self]()`) to the front of the explicit list. Its type may
be written explicitly (`fn F(self: Self)`) or omitted, in which case it
defaults to `Self` (`fn F(self)`, `fn F(ref self)`); `self` in the
implicit list is rejected.

Throughout checking, `self` is modeled as the first explicit parameter.
Because a method is just a function whose first parameter is `self`, it
can also be called as an ordinary function with the receiver passed
explicitly (`Type.M(obj, ...)`), not only as `obj.M(...)`. A new
`SemIR::CallArgParamPatterns` helper chooses the parameters matched
against the explicit arguments, excluding a leading `self` only when it
is supplied as a method-call receiver; arity checking, conversion, and
generic deduction use it. The resulting SemIR and lowering are
unchanged: `self` is still `call_param0`, and witnesses, thunks, and
vtables are unaffected.

An omitted `self` type is parsed as a `SelfBindingPattern` node with no
type expression; checking synthesizes the `Self` type so it behaves
exactly like `self: Self`. However, the exact spelling used must match
between a forward declaration and a definition, following #3763's rules
around declaration matching.

Generated functions, thunks, and C++ interop import/export build `self`
as the first explicit parameter, and the `self`-type override (e.g.
Derived->Base for a virtual override) applies to the explicit `self`.
Placement is validated by new diagnostics: `SelfInImplicitParamList`,
`SelfNotFirstParam`, and `SelfOutsideParamList`. The benchmark source
generator and the documentation adopt the `(self)` shorthand; the
prelude, the examples, and the test data are migrated in the following
commits.

Assisted-by: Claude Code with Claude Opus 4.7

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

docs/design/assignment.md

@@ -74,7 +74,7 @@ standard library.
 
 The operands of these operators can be any [expression](expressions/README.md).
 However, the first operand must be modifiable because it is passed to a
-`[ref self: Self]` parameter, which disallows most expression forms other than:
+`(ref self: Self)` parameter, which disallows most expression forms other than:
 
 -   The name of a `var` binding.
 -   A dereference of a pointer.

docs/design/generics/details.md

@@ -6630,7 +6630,7 @@ example, this `Set(T)` type may be compared to anything implementing the
 
 ```carbon
 class Set(T:! Ordered) {
-  fn Less[U:! Container with .ElementType = T, self: Self](u: U) -> bool;
+  fn Less[U:! Container with .ElementType = T](self, u: U) -> bool;
   // ...
 }
 ```
@@ -6648,7 +6648,7 @@ its elements implement the `Ordered` interface:
 ```carbon
 class DynArray(T:! type) {
   // `DynArray(T)` has a `Sort()` method if `T impls Ordered`.
-  fn Sort[C:! Ordered, ref self: DynArray(C)]();
+  fn Sort[C:! Ordered](ref self: DynArray(C));
 }
 ```