Include WordComponentParser.hh in WordParserImpl.hh

To avoid compilation error due to incomplete type.
Add locale to parser::Environment

The locale is mainly used to define character type categories and
string-number conversion.
Make Predicate argument Environment const

A predicate should not modify the argument environment, so it should be
const.
Add isTokenDelimiter function
Add isVariableNameChar function
Add CLocaleEnvironmentStub class
Add AssignmentParserImpl template class
Add AssignmentParser class
Add CommentSkipper class
Add CommandParser abstract class
Remove mSourceLocation from Command

It seems Command should have SourceBuffer::ConstIterator as a pointer to
the corresponding source location. For now we remove SourceLocation from
Command; we will add ConstIterator (or something else) again in the
future.
WIP: Add SimpleCommandParser class