Remove the number of functions that custom allocator users need to
provide; nobody should need to implement `substrdup`. Keep it to the
basics that are actually _needed_ for allocation (malloc, realloc,
free) and reimplement the rest ourselves.
In addition, move the failure check and error setting _out_ of the
custom allocators and into a wrapper so that users don't need to deal
with this. This also allows us to call our allocator (without the
wrapper) early so that it does not try to set an error on failure, which
may be important for bootstrapping.
Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.
The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.
Tie in the newly created infrastructure for swapping out memory
allocators into our settings code. A user can now simply use the new
option "GIT_OPT_SET_ALLOCATOR" with `git_libgit2_opts`, passing in an
already initialized allocator structure as vararg.
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.
