String: Convert string_builder.c to a cmod.
This is in preparation of moving String.Buffer from builtin.cmod.
Merge branch '8.1' into peter/travis
Runtime: Added some stack level paranoia.
Make sure that string_builder_vsprintf() doesn't
mess up the stack.
unsigned INT64 -> UINT64
Break out string_builder from stralloc.[ch] to string_builder.[ch].
More cleanuo of stralloc.[ch].