Enable GC at the "top level"

[shwestrick]

MPL currently avoids GC at the "top level" (i.e., outside any calls to par) because these collections typically interfere with parallelism. So far, for parallel programs, this hasn't been too much of a problem. But for sequential programs, this is obviously broken.

It's not hard to work around this issue by running the program inside an outermost par (see e.g. #112), but of course it would be nice if we didn't have to use such a workaround.

[shwestrick]

Note also that currently the GC is disabled until the scheduler boots up (i.e. it is disabled until $(SML_LIB)/basis/fork-join.mlb is loaded). Ideally, this initial part of the program should still be GC'ed with a simple sequential algorithm.