Currently, our memory allocators are being redirected to the correct
implementation at compile time by simply using macros. In order to make
them swappable at runtime, this commit reshuffles that by instead making
use of a global "git_allocator" structure, whose pointers are set up to
reference the allocator functions. Like this, it becomes easy to swap
out allocators by simply setting these function pointers.

In order to initialize a "git_allocator", our provided allocators
"stdalloc" and "crtdbg" both provide an init function. This is being
called to initialize a passed in allocator struct and set up its members
correctly.

No support is yet included to enable users of libgit2 to switch out the
memory allocator at a global level.