Add discrete interval encoding tree module to the standard library.

Discrete Interval Encoding Trees are a highly efficient set
implementation for fat sets.

This implementation is, in large parts, a translation of Caml Diet.
I have permission from the authors to license this version either under
the LGPL, or LGPL/BSD dual license.  We will choose the LGPL for
consistency with the rest of the standard library, but retain the other
option for the future.

library/diet.m:
	Add the new module.

library/library.m:
	Include the new module in the library.

NEWS:
	Announce the above addition.
Add enum.det_from_int.

Add a function enum.det_from_int, as requested by Julien and Zoltan.

library/enum.m:
library/diet.m:
	Move det_from_int from diet to enum.

tests/hard_coded/bitmap_simple.m:
	Remove redundant and conflicting definition.