Removed useless prototype and renamed object typecast functions
declaration macro.

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.

This unifies the object parse functions into one signature that
takes an odb_object.

This adds create and free callback to the git_objects_table so
that more of the creation and destruction of objects can be table
driven instead of using switch statements.  This also makes the
semantics of certain object creation functions consistent so that
we can make better use of function pointers.  This also fixes a
theoretical error case where an object allocation fails and we
end up storing NULL into the cache.

This uses the odb object accessors so we can change the internals
more easily...

This moves some of the odb_backend stuff that is related to the
internals of an odb_backend implementation into include/git2/sys.

Some of the stuff related to streaming I left in include/git2
because it seemed like it would be reasonably needed by a normal
user who wanted to stream objects into and out of the ODB.

Also, I added APIs for traversing the list of backends so that
some of the tests would not need to access ODB internals.

This adds crlf/lf conversion functions into buf_text with more
efficient implementations that bypass the high level buffer
functions.  They attempt to minimize the number of reallocations
done and they directly write the buffer data as needed if they
know that there is enough memory allocated to memcpy data.

Tests are added for these new functions.  The crlf.c code is
updated to use the new functions.

Removed the include of buf_text.h from filter.h and just include
it more narrowly in the places that need it.

This adds a check to the drop_crlf filter path to check it the
file in the index already has a CR in it, in which case this will
not drop the CRs from the workdir file contents.

This uncovered a "bug" in `git_blob_create_fromworkdir` where the
full path to the file was passed to look up the attributes instead
of the relative path from the working directory root.  This meant
that the check in the index for a pre-existing entry of the same
name was failing.