[MLton] mlbs in the top level of $(MLTON_ROOT)

[Stephen Weeks]

I'm wondering if it would be useful put an mlb for each library in the
top-level of $(MLTON_ROOT).  This would make the usual case (one main
library per directory) concise while still allowing other more
complicated uses.  For example, in mlyacc/src/sources.mlb, instead of

  $(MLTON_ROOT)/basis/basis.mlb
  $(MLTON_ROOT)/basis/unsafe.mlb
  $(MLTON_ROOT)/mlyacc-lib/mlyacc-lib.mlb

we would write

  $(MLTON_ROOT)/basis.mlb
  $(MLTON_ROOT)/basis/unsafe.mlb
  $(MLTON_ROOT)/mlyacc-lib.mlb

(we might even move unsafe to the toplevel, since it's historical that
it's in the basis)

Since the available directory/file names in $(MLTON_ROOT) is a shared
resource already, it doesn't seem to hurt to force people to avoid
.mlb name clashes in addition to directory name clashes.

Thoughts?