Rollup merge of #24132 - kwantam:master, r=alexcrichton

@mahkoh points out in #15628 that unicode.py does not use
normative data for Grapheme classes. This pr fixes that issue,
and does some minor cleanup of the unicode.py script.

In addition, GC_RegionalIndicator is renamed GC_Regional_Indicator
in order to stay in line with the Unicode class name definitions.
I have updated refs in u_str.rs, and verified that there are no
refs elsewhere in the codebase. However, in principle someone
using the unicode tables for their own purposes might see breakage
from this.