Simplify proxy_invoke and proxy_reflect

[mingxwa]

Motivation

In the current design, proxy_invoke() and the accessor of a convention type (specifically details::conv_impl) take the convention type as their template parameter. Similarly, proxy_reflect() and the accessor of a reflection type (specifically details::refl_impl`) also take the reflection type as their template parameter. The abuse of convention types and reflection types on these definitions makes their semantics more complex than necessary. Simplifying their definitions makes it easier to reason the code.

For example, for the following facade definition:

PRO_DEF_MEM_DISPATCH(MemFoo, Foo);

struct MyFacade : pro::facade_builder
    ::add_convention<MemFoo, void(), void(int)>
    ::build {};

Before this change, proxy_indirect_accessor<MyFacade> inherits the following two types:

MemFoo::accessor<MyFacade, pro::details::conv_impl<false, MemFoo, void(), void(int)>, void()>
MemFoo::accessor<MyFacade, pro::details::conv_impl<false, MemFoo, void(), void(int)>, void(int)>

After this change, the two types become:

MemFoo::accessor<MyFacade, false, MemFoo, void()>
MemFoo::accessor<MyFacade, false, MemFoo, void(int)>

Although this change has no runtime side effects, it can potentially improve compilation speed especially when a convention type has multiple overloads.

Changes

• Updated the definition of proxy_invoke. The first template parameter (convention type C) was replaced by a boolean flag (equivalent to prior C::is_direct) and a dispatch type (equivalent to prior typename C::dispatch_type).
• Updated the definition of proxy_reflect. The first template parameter (reflection type R) was replaced by a boolean flag (equivalent to prior R::is_direct) and a reflector type (equivalent to prior typename R::reflector_type).
• Updated the definition of the accessors in the library.
• Revised the implementation of concept facade<F> to detect correctness of each reflection type defined in typename F::reflection_types. Also fixed the corresponding test case in proxy_traits_tests.cpp.
• Revised the semantics of the ProAccessible requirements.
• Updated unit tests accordingly.
• Updated spec accordingly.