Wrote int2str as FASTCALL-compliant; untested

Wrote the int2str integer-to-character string conversion function. This function is written as FASTCALL-compliant; that is, the first four arguments of any call are passed forward in the GPRs (%EAX-%EDX). This code is recursive and uses %EBX as a storage for a push address to memory of a character. 

Once revised, the calls will instead use %DL as an absolute-type-safe method of holding the byte, as using %EDX carries the possibility of character errors and buffer overwrites.

Note that FASTCALL_INT2STR is marked as global to allow for export for C/C++ APIs, should that be the preferred route.