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.

src/core/pp.c

@@ -30,7 +30,7 @@
 #endif
 
 /* Implements a pretty printer for Janet. The pretty printer
- * is farily simple and not that flexible, but fast. */
+ * is simple and not that flexible, but fast. */
 
 /* Temporary buffer size */
 #define BUFSIZE 64