Users can define `GIT_DEPRECATE_HARD` if they want to remove all
functions that we've "softly" deprecated.

Add `@deprecated` to the functions that are, so that they'll appear that
way in docurium.

`git_stream_register_tls` is now deprecated; mark it in an if guard with
the deprecation.

This should not be included in `deprecated.h` since it is an uncommonly
used `sys` header file.

Replace the `GITERR` values with a `const int` to deprecate error
values.

Move to the `git_error` name in error-related functions, deprecating the
`giterr` functions.  This means, for example, that `giterr_last` is now
`git_error_last`.  The old names are retained for compatibility.

This only updates the public API; internal API and function usage
remains unchanged.

The C standard does not specify whether an enum is a signed or unsigned
type.  Obviously, any enum that includes negative values _must_ be
signed, but if all values are positive then the compiler is free to
choose signed or unsigned.

Thus, by changing the type signatures to `git_object_t` and declaring
the old `GIT_OBJ_` values as a signed or unsigned int, we risk a
mismatch between what the compiler has chosen for a `git_object_t`'s
type and our type declaration.

Thus, we declare the deprecated values as the enum instead of guessing.