Non-tail return

Non-tail return

[Sanghyeon Seo]

Richard Jones suggested that type-safe return statement which
immediately returns from the innermost function would be useful. So I
developed an implementation. His sample code (from Extlib) works
as-is.

http://sparcs.kaist.ac.kr/~tinuviel/devel/ocaml/

Some notes:

* This depends on nontail exit patch I did as a part of break/continue
patch. break-continue-v2 was broken to break-continue-v3 and
nontail-exit-v1.
* A stack of function's return type is maintained. "return e" causes
unification of "e" and return type on the top of the stack. "return e"
itself is typed as unit.
* Function is translated with implicit catch around body, and return
is translated to raise. Simplification removes implicit catch when no
return is used, so existing codes should compile same.

-- 
Seo Sanghyeon