Fix #339. Revamp memory handlign in Boost.SIMD

This patch performs the following :

 * Move memory to be a sub-component of boost.simd
 * Restructured memory related functions placement and documentation
   so that all memory related functions and metafunctions are separate
   from operators
 * Documentation structure have been amended to support this new
   structure and a seminal documentation has been written
 * Added a low level malloc like API in addition to allocate API
 * Allocate can now use statically specified alignment
 * Fixed reallocate code when resizing down
 * Implement stack based buffer declaration macro
 * Added global new/delete overload to use SIMD allocator