Major change:

Major change:

- New drifting/undrifting code and closure analysis.

Minor changes:

- lap:make-uncoditional-branch now returns a sequence of instructions,
rather than a single instruction.

- Uses of `expt' in machin and lapgen have temporarily been commented
out because the `usual-definition' declaration does not work when
`compile-by-procedures?' is turned on.

- letrec-value now gives only a warning when binding a trivial
closure.

- close-procedure! in blktyp now only gives a warning when a
non-trivial closure becomes trivial.