[commit] string boolean

I wrote stuff for string boolean tests, to be the same as perl...

Hopefully I've put the bits in the right places this time.

 * Added get_digit() to chartypes
 * Added string_bool() to strings
 * Added if_s_ic op, to use same
 * Added (PerlString)->get_bool stuff to perlstring.pmc
 * Tests for above, also a for couple of other ops without tests

The unicode chartype has only had placeholders added for get_digit,
string_to_num has been updated to use type->get_digit rather than the
previous ascii-isms.  Some remain, but we need to carefully define
what we mean by a number before we let thousands of code points act as
a minus sign.

Alex Gough

• [commit] string boolean by Alex Gough