make nqp:chr barf when code points invalid or out of range
add bootstrap files
Fallback for box/unbox in code-gen.

Discovered while using the toolchain to build a language other than
NQP/Perl 6.
Merge branch 'master' of git://github.com/dwarring/nqp
Formatting cleanups.
Ensure 63-slurp.t works on Windows also.