[MLton] caching elaboration

[Matthew Fluet]

> So, my idea is for MLton, as it elaborates, to record the facts about
> the environment that it depends on, and then to save the world at some
> point during elaboration.

I'll note that the proposal seems to point towards a scheme where
elaboration prompts lexing/parsing.  That seems fine (and it will probably
make the lexer/parser for MLBs simpler), but it will change the behavior
of the compiler.  In particular, it will no longer be the case that
all elaboration type-errors will occur after all syntax errors.