Also adds the janet_lengthv API call. This is
needed because janet_length returns a 32 bit integer, where
as lengthv lets us return larger values (useful for typed arrays).
janet_mcall is an api function that should make it easier to call
a janet method from C code. It shares a similar signature with
janet_call.
The break special form can break out of both loops
and functions with an early (nil) return. Mainly useful
for generated code in macros, and should probably be discouraged
in user written code.
