enclose warnings.h generation in a block

...to limit the number of variables visible everywhere and
make it a bit easier to see what I am doing as I refactor
regen/warnings.pl
replace printTree with warningsTree

we return, rather than print, the warnings, so we can potentially
futz around with the string and put it where we like without
having to worry about C<select>
merge most of perllexwarn into warnings
turn perllexwarn into a stub
regen/warnings.pl no longer touches perllexwarn
remove references to perllexwarn from warnings.pm
replace links to perllexwarn with links to warnings

or, sometimes, simply remove them
bump the version of warnings.pm

(and of regen/warnings.pl)
warnings.pm: improve awkward sentence in pod
do not podcheck regen/warnings.pl
regenerate warnings.pm
Merge perllexwarn.pod into warnings.pm