Implement indirection syntax

[mingxwa]

Changes

• Whether a dispatch type D applies to a pointer, or the target of a pointer, is now determined by D::is_direct (required to be provided as static constexpr bool). In the previous design, this was determined by overload resolution, which makes the semantics ambiguous.
• Similar with dispatch, a reflection type R also requires R::is_direct to be provided as static constexpr bool.
• The direct and indirect accessors are split. Direct accessors are inherited by a proxy, while indirect accessors are available via proxy::operator-> or proxy::operator*.
• Added another 12 macros to define direct or indirect dispatch types, while the previous ones (like PRO_DEF_MEM_DISPATCH) defaults to indirect:

#define PRO_DEF_INDIRECT_MEM_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_DIRECT_MEM_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_INDIRECT_FREE_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_DIRECT_FREE_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_INDIRECT_OPERATOR_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_DIRECT_OPERATOR_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_INDIRECT_PREFIX_OPERATOR_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_DIRECT_PREFIX_OPERATOR_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_INDIRECT_POSTFIX_OPERATOR_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_DIRECT_POSTFIX_OPERATOR_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_INDIRECT_CONVERSION_DISPATCH(__NAME, ...) // See proxy.h
#define PRO_DEF_DIRECT_CONVERSION_DISPATCH(__NAME, ...) // See proxy.h

• Updated unit tests and README accordingly.