Misc cleanups:
Prepare beast for switching to libc++'s c++14 in the future.
Guard beast's std::equal with BEAST_NO_CXX14_COMPATIBILITY.
Remove ineffectual 'const' from two places where it was
having no impact: declaring an alias to a reference const.
Consider renaming SField::ref to cref.
Distribute namespace beast through headers in beast_core.h
Distribute namespace beast through src's in beast_core.cpp
War on using declarations.