Improve proxy_invoke

[mingxwa]

Changes

• In proxy_invoke, pass the reference of proxy instead of the reference of proxy::ptr_ to the indirect function pointer. With this change, the assembly generated from caller side could be further optimized.

For example, for the code presented in section 5.1.3, P3086R3:

PRO_DEF_MEM_DISPATCH(MemDraw, Draw);
struct FDrawable : facade_builder
    ::add_convention<MemDraw, void() const>
    ::build {};

p->Draw();  // p is a value of proxy<FDrawable>

Here is a comparison table of the generated assembly before and after this change:

Compiler	Before	After
x86-64 (AMD64) Clang 19.1.0	mov rax, qword ptr [rdi] add rdi, 8 jmp qword ptr [rax + 16]	mov rax, qword ptr [rdi] jmp qword ptr [rax + 16]
ARM64 Clang 19.1.0	ldr x8, [x0], #8 ldr x1, [x8, #16] br x1	ldr x8, [x0] ldr x1, [x8, #16] br x1
RISC-V RV64 Clang 19.1.0	ld a1, 0(a0) ld a5, 16(a1) addi a0, a0, 8 jr a5	ld a1, 0(a0) ld a5, 16(a1) jr a5

• The fallback of a weak dispatch is no longer nullptr, but proxy or proxy_indirect_accessor. This allows invoking other proxy functions from within a dispatch, similar to a default method in C# or Java.
• Removed PRO_DEF_WEAK_DISPATCH (deprecated since 3.2.0).
• Updated the implementation of the built-in dispatch types accordingly.
• Updated unit tests accordingly.

Spec will be updated separately

[tian-lt]

LGTM