Include version in released source files

Include version in released source files

I realised that determining which Monocypher version was used in a
project was not trivial. We could look at Monocypher's code and deduce
the release, but that's tedious and error prone. So I've made those
versions more explicit:

- Source and header files begin by a comment describing the version.
- The pkg-config file created by `make install` include that version.
- The version number of unreleased code (under git) is "__git__"
- The version number of released code is whatever `git describe --tags`
  tells us.
- the "tarball" target in the makefile was changed to the more standard
  "dist".

To release a new version, we just add a tag, then call `makefile dist`.
The version of the released source file will appear at a glance, right
there on the first line.

Note: the release process blindly replaces all instances of "__git__" by
the suitable version number.  This could be used to version things other
than comments, like string constants.