Rename MCField::settext to settext_oldstring

The MCString variant of MCField::settext has been renamed to MCField::settext_oldstring. In its place, the new MCField::settext takes a StringRef.
Merge pull request #155 from runrevfraser/refactor-syntax_unicode_field_clean

Rename MCField::settext to settext_oldstring
Merge branch 'refactor-syntax_unicode' into refactor-syntax_unicode_object

Merge with upstream branch before pull into upstream.