Replace compile tests that check if types are copy constructible and copy assignable with normal tests that use type_traits (if available).

is_copy_assignable is broken in MSVC.

Array is split into an abstract Buffer class and a concrete MemoryBuffer class. BasicWriter now does all memory allocation through a Buffer object. Subclasses of BasicWriter may use different buffer types. The new BasicMemoryBuffer class uses the default MemoryBuffer.