Improve diagnostics by reducing constraints

[mingxwa]

Changes

• Added API template <class T, class F> concept proxiable_target: concept for make_proxy() and allocate_proxy.
• Added diagnostics to proxy::proxy, proxy::operator=, proxy::emplace, make_proxy and allocate_proxy, and removed proxiable constraints from these APIs, resolves make_proxy shall have more strict constraints #247.
• Forbid creation of proxy from another proxy of a different facade type.
• Reverted Bypass proxiability check in unresolved context #255 since it's no longer necessary.
• Removed constraints from the constructors of internal pointer types to reduce complexity.
• Added some unit tests.

[tian-lt]

LGTM