[MLton] mlnlffigen is awesome (was: bootstrap problem)

[Stephen Weeks]

Matthew's and Henry's points make perfect sense.  I'm fine with
long-style names like

  $(SML_LIB)/smlnj-lib/HTML/html-lib.mlb

Clarity is much more important than concision in MLBs, since they are
such a small fraction of the code in a project.

One remaining question in my mind is whether
$(HOME)/.mlton/mlb-path-map should be processed by default.  On the
minus side, it is an implicit dependence on an environment variable
and makes it easier for package distributors to make mistakes,
accidentally depending on personal path variables.  But, if
distributors are aware of it perhaps that's not such a big deal.  On
the plus side is the possibility that most users would find it
convenient.  Since the theory is that there are many more users than
distributors, we should favor them.  While writing a script like
Matthew's is easy, it's kind of silly to make everyone do it.

Another question is whether to allow multiple -mlb-path-map arguments?
And if so, does it make more sense to have later definitions take
precedence (as we have) or earlier ones?