Fix empty fragment being created when an access modifier occurs by itself

[bamarsha]

Access modifiers were registered as a fragment header, but no parser existed that could parse them by themselves (they could only be parsed if they were followed by function, operation, or newtype). This means that if they aren't followed by one of those headers, none of the fragment parsers would succeed, so the default parser would try to create an invalid fragment by skipping over input until it finds the next fragment header. However, the next fragment header was the access modifier itself, so it wouldn't skip any input at all, creating an empty invalid fragment instead and causing this exception:

QsCompilerException: fragment has been built but no input was consumed

To fix this I added parsers for the access modifiers that can consume just an access modifier by itself before creating an invalid fragment, which makes sure that input is always consumed when a fragment is built.

See #259.