add more type predicates.

README.md

@@ -88,19 +88,24 @@ The following predicates are built-in to rusty-wam.
 * `between/3`
 * `call/1..63`
 * `catch/3`
+* `compound/1`
 * `display/1`
 * `duplicate_term/2`
 * `false/0`
+* `float/1`
 * `functor/3`
 * `integer/1`
 * `length/2`
 * `maplist/1..8`
 * `member/2`
 * `memberchk/2`
+* `nonvar/1`
 * `once/1`
+* `rational/1`
 * `reverse/2`
 * `select/3`
 * `setup_call_cleanup/3`
+* `string/1`
 * `throw/1`
 * `true/0`
 * `var/1`