RFC: use _perl_core_constants

I was taught as an undergraduate that there are only three valid numbers 
in computer science: 0, 1, and as many as you like.  Every other numeric 
constant needs a mnemonic or an explanation.

I believe that, and hence it pains me to see various modules use 
constants that are needed to communicate with the Perl core, but are not 
automatically tied to the core.  I therefore propose this module which 
would be quite simple, actually, and would merely export various Perl 
core constants that are deemed to be useful for modules to have.

Initially it would have the names for those used in $^H.

Better ideas for its name are welcome.  I began it with an underscore to 
underscore its intended private use.

• RFC: use _perl_core_constants by Karl Williamson