Make sure to beta-reduce "let v1 = v2 in e".

I've only spotted terms on this form when using explicit force/shares, but the
current situation is that the rest of the optimizations in the compiler fail
to match with these junk terms in the AST.
Merge pull request #54 from pjonsson/betareduce-more

Make sure to beta-reduce "let v1 = v2 in e".