Add Unicode support to MCField

MCField now uses an MCStringRef to store its label.
Simplify MCField label handling

As per Mark's suggestions, simplified the label handling for MCField (the label should now never be set to nil, only the empty string).
Removed un-necessary checks for nil
Merge pull request #153 from runrevfraser/refactor-syntax_unicode_field_clean

Refactor syntax unicode field clean