Use a HEK to store the GV's name, rather than a malloc()ed string.

Use a HEK to store the GV's name, rather than a malloc()ed string.
Saves 1 word in each GV (no more strlen), and will also save the memory
used by the string itself, as the HEK will exist already due to the
key used by the symbol table for this GV.

p4raw-id: //depot/perl@27379