Greatly limit the debug verbosity when parsing strings.

Previously, all possible parse instances of a string were shown, which
included the unquoted, single quoted, and double quoted versions,
meaning that "always" was shown as:

    string(([ws] & ('always'
		    | ("'" & ['a', 'l', 'w', 'a', 'y', 's'] & "'")
		    | ('"' & [rich_char(c='a'), rich_char(c='l'), rich_char(c='w'), rich_char(c='a'), rich_char(c='y'), rich_char(c='s')] & '"'))))

This changes the output to:

    string([frequency])