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.

src/libostree/libostree-devel.sym

@@ -22,12 +22,17 @@
    - uncomment the include in Makefile-libostree.am
 */
 
+LIBOSTREE_2021.2 {
+global:
+  ostree_repo_write_regfile_inline;
+} LIBOSTREE_2021.1;
+
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the year.  This is just a copy/paste
  * source.  Replace $LASTSTABLE with the last stable version, and $NEWVERSION
  * with whatever the next version with new symbols will be.
-LIBOSTREE_2020.$NEWVERSION {
+LIBOSTREE_2021.$NEWVERSION {
 global:
   someostree_symbol_deleteme;
-} LIBOSTREE_2020.$LASTSTABLE;
+} LIBOSTREE_2021.$LASTSTABLE;
 */