restrict legal identifier names for security

Accepting any combination of legal Unicode Identifier characters has led 
to security problems.  Hence Unicode has added guidance that we are not 
following.

I'm proposing that the PSC and other interested parties familiarize 
yourselves with https://www.unicode.org/reports/tr39/ "Unicode Security 
Mechanisms" and https://www.unicode.org/reports/tr55/ "Unicode Source 
Code Handling" so that we can discuss which we might want to implement.
I think it's a no brainer that we stop accept deprecated characters 
(there are 20-ish of these).  But there's much more there.

The benefits are fewer potential security holes, including many none of 
us on this project has the background to be aware of.  We would be using 
accumulated knowledge from bitter experiences of others.  This could 
detect existing trojans and we could get them removed.

The downside is we might break existing legitimate code.  The more 
restrictions we impose, the more likely there is breakage.  This could 
be alleviated by enabling some restrictions only under a 'use v5.xx'