stringnull branch

[Peter Lobsinger]

Hi,

I've created the stringnull branch to use STRINGNULL in stead of NULL
as the representation of the null string. It is now passing all tests
(plus a few I added along the way), and all major HLLs (rakudo,
cardinal, pipp, pynie, partcl) are the same status they are against
trunk (with a patch against Rakudo).

I'm looking to merge this soon (24-hours-ish). Any objections?

Peter Lobsinger