consider `decl != NULL` case
Tweak code style
Tweak token consumers

parseImport should return parsed `Import`s.
Reduce use of variable 'a'
Parse all prefix attributes inside while loop

diagnostic
- just only disable prefix method attributes on module-level functions
- better error message for static static this(), shared static this() shared, etc.

enh
- warning for prefix method attributes (eg. const)
- use prefix const qualifier for return type inference and qualifying return-type
+linkage
+Tweak pLastDecl on for test/compilable/ddocunittest.d