ocaml: phase 6 String extensions (+6 tests, 406 total)

ends_with, contains, trim, split_on_char, replace_all, index_of —
wrap host SX primitives via new _string_* builtins. String module
now substantively covers OCaml's Stdlib.String.

lib/ocaml/runtime.sx

@@ -326,6 +326,12 @@
      let uppercase_ascii s = _string_upper s
      let lowercase_ascii s = _string_lower s
      let starts_with prefix s = _string_starts_with prefix s
+     let ends_with suffix s = _string_ends_with suffix s
+     let contains s sub = _string_contains s sub
+     let trim s = _string_trim s
+     let split_on_char c s = _string_split_on_char c s
+     let replace_all s a b = _string_replace s a b
+     let index_of s sub = _string_index_of s sub
    end ;;
 
    module Char = struct