Add parsing for access modifiers using alternative syntax

[bamarsha]

This adds parsing using the syntax where the access modifier comes after the fragment header and before the symbol. For example, operation private Foo () : Unit. The syntax will be changed to private operation Foo () : Unit in another PR.

See #259.

[bettinaheim]

Approving this without detailed review since this code will be revised significantly and the PR only serves the purpose of preserving this as an intermediate state in the history.