repo: Add ostree_repo_write_regfile_inline

When working on ostree-ext and importing from tar, it's
quite inefficient and awkward for small files to end up creating
a whole `GInputStream` and `GFileInfo` and etc. for small files.

Plus the gtk-rs binding API to map from `impl Read` to Gio
https://docs.rs/gio/0.9.1/gio/struct.ReadInputStream.html
requires that the input stream is `Send` but the Rust `tar` API
isn't.

This is only 1/3 of the problem; we also need similar APIs
to directly create a symlink, and to stream large objects via
a push-based API.

apidoc/ostree-sections.txt

@@ -349,6 +349,7 @@ ostree_repo_write_metadata
 ostree_repo_write_metadata_async
 ostree_repo_write_metadata_finish
 ostree_repo_write_content
+ostree_repo_write_regfile_inline
 ostree_repo_write_metadata_trusted
 ostree_repo_write_metadata_stream_trusted
 ostree_repo_write_content_trusted