Currently we only use it only for ‘since’ annotations but with these
patches it should be fairly simple to add new attributes if we wish to.

Closes #26. It seems to work fine but due to 7.10 rush I don't have the
chance to do more exhaustive testing right now. The way the meta is
output (emphasis at the end of the whole comment) is fairly arbitrary
and subject to bikeshedding.

Note that this makes test for Bug310 fail due to interface version bump:
it can't find the docs for base with this interface version so it fails.
There is not much we can do to help this because it tests for ’built-in’
identifier, not something we can provide ourselves.