ocaml: phase 6 String/Char/Int/Float/Printf modules (+13 tests, 278 total)

Host primitives _string_length / _string_sub / _char_code / etc. exposed
in the base env (underscore-prefixed to avoid user clash). lib/ocaml/
runtime.sx wraps them into OCaml-syntax modules: String (length, get,
sub, concat, uppercase/lowercase_ascii, starts_with), Char (code, chr,
lowercase/uppercase_ascii), Int (to_string, of_string, abs, max, min),
Float.to_string, Printf stubs.

Also added print_string / print_endline / print_int IO builtins.

lib/ocaml/runtime.sx

@@ -136,6 +136,40 @@
        match r with
        | Ok _ -> false
        | Error _ -> true
+   end ;;
+
+   module String = struct
+     let length s = _string_length s
+     let get s i = _string_get s i
+     let sub s i n = _string_sub s i n
+     let concat sep xs = _string_concat sep xs
+     let uppercase_ascii s = _string_upper s
+     let lowercase_ascii s = _string_lower s
+     let starts_with prefix s = _string_starts_with prefix s
+   end ;;
+
+   module Char = struct
+     let code c = _char_code c
+     let chr n = _char_chr n
+     let lowercase_ascii c = _string_lower c
+     let uppercase_ascii c = _string_upper c
+   end ;;
+
+   module Int = struct
+     let to_string i = _string_of_int i
+     let of_string s = _int_of_string s
+     let abs n = if n < 0 then 0 - n else n
+     let max a b = if a > b then a else b
+     let min a b = if a < b then a else b
+   end ;;
+
+   module Float = struct
+     let to_string f = _string_of_float f
+   end ;;
+
+   module Printf = struct
+     let sprintf fmt = fmt
+     let printf fmt = print_string fmt
    end")
 
 (define ocaml-stdlib-loaded false)