Use :length method for (length abstract)

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.

test/suite5.janet

@@ -54,6 +54,7 @@
 (assert (= (a 2) (b 1) ) "tarray views pointing same buffer")
 (assert (= ((tarray/slice b) 3) (b 3) (a 6) 6) "tarray slice")
 (assert (= ((tarray/slice b 1) 2) (b 3) (a 6) 6) "tarray slice")
+(assert (= (:length a) (length a)) "length method and function")
 
 (assert (= ((unmarshal (marshal b)) 3) (b 3)) "marshal")