Invoke through an erased context instead of the proxy type (#68)

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.

Introduce erased_context, which carries only a pointer to the storage of
the contained value, and type invokers as invoker<Ctx, O>. Invocation
now resolves the contained type inside the context rather than through
reinterpret_invoke, and 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.

Declare ptr_ 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. Where a proxy
is invoked more than once, the caller then 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. Neither sizeof(proxy) nor its alignment changes.

relocate_dispatch becomes a tag with its own erased_context
specialization, which absorbs the bitwise-relocation path that used to
be selected via internal_dispatch, and internal_dispatch is removed.
substitution_dispatch needs the same path, because it relocates a
bitwise-relocatable value without requiring it to be move-constructible,
so it gets a specialization too; both it and the specialization drop out
once `super` replaces substitution.

Removes the public reinterpret_invoke, which had no remaining use: the
invoker macro was its only caller inside the library.

Author: mingxwa

docs/spec/proxy/.pages

@@ -9,7 +9,6 @@ nav:
   - operator= (non-member): friend_operator_equality.md
   - operator=: assignment.md
   - reflect (non-member): friend_reflect.md
-  - reinterpret_invoke (non-member): friend_reinterpret_invoke.md
   - reset: reset.md
   - swap (non-member): friend_swap.md
   - swap: swap.md

docs/spec/proxy/README.md

@@ -47,7 +47,6 @@ As per `facade<F>`, `typename F::convention_types` shall be a [tuple-like](https
 | [`swap`](friend_swap.md)                             | overload the [`std::swap`](https://en.cppreference.com/w/cpp/algorithm/swap) algorithm |
 | [`invoke`](friend_invoke.md)                         | invokes a `proxy` with a specified convention                |
 | [`reflect`](friend_reflect.md)                       | acquires reflection information of a contained type          |
-| [`reinterpret_invoke`](friend_reinterpret_invoke.md) | invokes a dispatch on a `proxy` whose contained type is known statically |
 
 ## Comparing with Other Standard Polymorphic Wrappers
 

docs/spec/proxy/friend_invoke.md

@@ -60,4 +60,3 @@ int main() {
 ## See Also
 
 - [function template `reflect` (`proxy<F>`)](friend_reflect.md)
-- [function template `reinterpret_invoke` (`proxy<F>`)](friend_reinterpret_invoke.md)

docs/spec/proxy/friend_reinterpret_invoke.md

@@ -2,4 +2,3 @@ nav:
   - proxy_indirect_accessor: README.md
   - invoke: friend_invoke.md
   - reflect: friend_reflect.md
-  - reinterpret_invoke: friend_reinterpret_invoke.md

docs/spec/proxy_indirect_accessor/.pages

@@ -29,7 +29,6 @@ Class template `proxy_indirect_accessor` provides indirection accessibility for
 | ---------------------------------------------------- | ------------------------------------------------------------ |
 | [`invoke`](friend_invoke.md)                         | invokes a `proxy` with a specified convention                |
 | [`reflect`](friend_reflect.md)                       | acquires reflection information of a contained type          |
-| [`reinterpret_invoke`](friend_reinterpret_invoke.md) | invokes a dispatch on a `proxy` whose contained type is known statically |
 
 ## See also
 

docs/spec/proxy_indirect_accessor/README.md

@@ -59,4 +59,3 @@ int main() {
 ## See Also
 
 - [function template `reflect` (`proxy_indirect_accessor<F>`)](friend_reflect.md)
-- [function template `reinterpret_invoke` (`proxy_indirect_accessor<F>`)](friend_reinterpret_invoke.md)