[super! 3/6] Invoke through an erased context instead of the proxy type

[mingxwa]

An invoker's erased function pointer was typed R (*)(proxy<F>&, D, Args...), so its type named the facade it was built for. That is fine while metadata is only ever read by a proxy of exactly that facade, but it prevents one proxy from reusing another's invoker: two proxy types have different layouts, so passing one where the other is expected is not valid. Reusing a base facade's invokers is what super (more PRs coming ahead) needs.

• Adds erased_context, which carries only a pointer to the storage of the contained value. Invokers become invoker<Ctx, O>.
• The lifetime dispatches take a void* rather than a target proxy, so their invokers no longer mention F either.
• Resetting the source metadata after an rvalue-qualified call moves to the call site, where the facade is known.
• relocate_dispatch becomes a tag with its own erased_context specialization, which absorbs the bitwise-relocation path that used to be selected through internal_dispatch. internal_dispatch is removed.
• ptr_ is declared ahead of meta_ so that the storage of the contained value lies at offset 0. Every invocation now builds a context from it, and at offset 0 that context is the address of the proxy itself.

Declaring ptr_ first is what keeps the new indirection free at the call site. Where a proxy is invoked more than once, the caller keeps a single value live instead of a separate context pointer. On aarch64 that removes a spill and reload of the context across the first call and shrinks the frame by 16 bytes:

 stp  x29, x30, [sp, #-48]!        stp  x29, x30, [sp, #-32]!
 ...                               ...
 ldr  x1, [x0], #8                 ldr  x1, [x0, #16]
 str  x0, [sp, #40]   <- spill     (none)
 blr  x1                           blr  x1
 ldr  x0, [sp, #40]   <- reload    mov  x0, x19

substitution_dispatch needs the same relocation path, because it relocates a bitwise-relocatable value without requiring it to be move-constructible, so it gets a specialization too. That specialization will be removed together with substitution_dispatch in the last PR of this stack.

Breaking change. Removes the public reinterpret_invoke.