RFR: 8249566: jshell tool: retained modes from JDK-13 or prior cause confusing messages to be generated for records

[Robert Field]

Please review....

Fixes the corruption of JDK-13 or earlier user defined modes and 
switches to a textual format resistant to future corruption.  The issue 
in the bug title is just one resulting error (the mode is deeply corrupted).

Fixing this necessitated a rewrite of all the code related to processing 
selectors, this was put in a new source file Selector.java.  It is a lot 
of code, but I've documented thoroughly.

I've hand tested by using JDK-13 and JDK-14 retained modes, can't really 
test the future language thing ;-)
and none of that is reasonably testable with the test harness. One of 
the side-effects of the rewrite is that the user's layout of a selector, 
is preserved, not rewritten in fixed order, so I needed to update one 
test case accordingly.

Issue:

https://bugs.openjdk.java.net/browse/JDK-8249566

Webrev:

http://cr.openjdk.java.net/~rfield/8249566v0.webrev/

Thanks,
Robert