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
Fixed reallocate code when resizing down
Missing load in store test
Fix alignment base on ARM with respect to iOS
Implement stack based buffer declaration macro
Added global new/delete overload to use SIMD allocator
Extended new/delete support for aligned allocation
Revamped allocation/deallcoation system
Fix for stack_buffer error
Various fixes in alloc* related files
Actually allcoates the strict minimum in case of stashing
Fix realloc definition w/r to system definition