* src/typecheck.ml: Actually implement a simple conv_p

* src/subst.ml: New file.
* src/lexp.ml: Use it.
(lexp): Remove UnknownType.

* src/util.ml (SMap): Move from grammar.ml
* src/grammar.ml (SMap): Move to util.ml

* GNUmakefile (OBFLAGS): Rename from CFLAG.  Remove -rectypes.

* src/lparse.ml (dlxp, dltype): Use arbitrary expression instead of
UnknownType.

* src/main.ml: Use named import rather than `open`.