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/eval.sx

@@ -47,7 +47,26 @@
       ;; can pattern-match them.
       (list "raise" (fn (e) (raise e)))
       (list "failwith" (fn (msg) (raise (list "Failure" msg))))
-      (list "invalid_arg" (fn (msg) (raise (list "Invalid_argument" msg)))))))
+      (list "invalid_arg" (fn (msg) (raise (list "Invalid_argument" msg)))
+      )
+      ;; Host primitives exposed for the OCaml stdlib (lib/ocaml/runtime.sx).
+      ;; Underscore-prefixed to avoid clashing with user names.
+      (list "_string_length" (fn (s) (len s)))
+      (list "_string_get"    (fn (s) (fn (i) (nth s i))))
+      (list "_string_sub"    (fn (s) (fn (i) (fn (n) (slice s i (+ i n))))))
+      (list "_string_concat" (fn (sep) (fn (xs) (join sep xs))))
+      (list "_string_upper"  (fn (s) (upper s)))
+      (list "_string_lower"  (fn (s) (lower s)))
+      (list "_string_starts_with" (fn (p) (fn (s) (starts-with? s p))))
+      (list "_int_of_string" (fn (s) (parse-number s)))
+      (list "_string_of_int" (fn (i) (str i)))
+      (list "_string_of_float" (fn (f) (str f)))
+      (list "_char_code" (fn (c) (char-code c)))
+      (list "_char_chr"  (fn (n) (char-from-code n)))
+      ;; Print: prints to host stdout via println.
+      (list "print_string"  (fn (s) (begin (print s) nil)))
+      (list "print_endline" (fn (s) (begin (println s) nil)))
+      (list "print_int"     (fn (i) (begin (print (str i)) nil))))))
 
 (define ocaml-env-lookup
   (fn (env name)