Rename Source::FileParser to just Parser.

This is really the main parsing class and shouldn't be buried deep
inside source/ dir.
Create MiniParser helper for use in parsing tests.

This helps to keep the parsing setup in one place and allow easier
changes to it when needed.  Each spec file only needs to call the
Helper::MiniParser.parse with a few arguments, simplifying the
setup code inside actual spec files.

Because the MiniParser.parse always returns Relations object, add
a #length method to Relations class, so that the tests that previously
checked the length of simple classes hash can now call the same method
on Relations object and continue passing.
Move use of Parser out of Source::File.

With the help of our new MiniParser, we only need to change the
parsing setup code in there, not touching all the related specs.
Remove return value from generated setters.

The missing return value is now just nil, not "undefined".