Dlang parser improvements

Added 'dkind' constructs for the D language parser. This allows better customisation of the parser, similar to the Java one. I've also beefed up the simple D unit test.
No, Geany has a different hack (I'm actually fixing some of them now) to parse it as a namespace. I'm talking about your first PR:
https://github.com/fishman/ctags/pull/133/files#diff-9c07e5e602aac928d6d779592925a132R1888
https://github.com/fishman/ctags/pull/133/files#diff-9c07e5e602aac928d6d779592925a132R886

Looks like it should really add & use `DECL_MODULE, TAG_MODULE` instead.