Added two new traits and a way to specify them

Added two new traits and a way to specify them

Summary:
Added IsBits (has value semantics; can be copied by memcpy and
deleted by free. Implies IsRelocatable) and Is0Initializable (new (ptr)
T() is equivalent to memset(ptr, 0, sizeof(T))).
Converted the boost types for IsRelocatable to std types (now available
from <type_traits>).
Added a new way to specify IsRelocatable, IsBits, and Is0Initializable:
typedef std::true_type <property-name> in your class. No namespace
exiting required. This method also supports inheritance, and may be
overriden in base classes.
Added a test file to test Traits.

Test Plan:
Run new test file. Compile some real-world code (in my case,
multifeed).

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D610996