Remove some macros

[mingxwa]

Changes

• Replaced macro PRO_DEF_OPERATOR_DISPATCH, PRO_DEF_LHS_OPERATOR_DISPATCH and PRO_DEF_RHS_OPERATOR_DISPATCH with class template pro::operator_dispatch<fixed-string Sign, bool Rhs = false>.
• Replaced macro PRO_DEF_CONVERSION_DISPATCH with class template pro::conversion_dispatch<class T, bool Expl = true>.
• Removed the 4th parameter in macro PRO_DEF_MEM_DISPATCH and PRO_DEF_FREE_DISPATCH to define a default implementation.
• Added another macro PRO_DEF_WEAK_DISPATCH to explicitly extend an existing dispatch type with a default implementation.
• Updated unit tests and README accordingly.

[tian-lt]

LGTM.