Feature request: support for local

[hai-nguyen-van]

Thanks for the great effort of supporting .mlb files. It would be great to integrate the syntax local ... in ... end if you ever have time. For instance, it can be useful to use the MLton.Random structure:

 local $(SML_LIB)/smlnj-lib/Util/smlnj-lib.mlb in
   signature RAND
   structure Rand
 end
 local $(SML_LIB)/basis/mlton.mlb in
   signature MLTON_RANDOM
   structure MLton
 end