lib: Add a private copy of checksum-instream

lib: Add a private copy of checksum-instream

The current `OstreeChecksumInputStream` is public due to a historical
mistake.  I'd like to add an OpenSSL checksum backend, but that's
harder without breaking this API.

Let's ignore it and create a new private version, so it's easier to do the
GLib/OpenSSL abstraction in one place.

Closes: #738
Approved by: jlebon