Changed bignum_to_double to produce a correctly rounded result. The

Changed bignum_to_double to produce a correctly rounded result.  The
code is a lot hairier since it implements IEEE style round to even,
but it is not much slower than the old `accumulate' loop since it
rarely has to look at more that two or three words.

The new version no longer signals an floating point overflow error on

	(INTEGER->FLONUM (- (EXPT 2 1024) 1) #b10)

This means that (INTEGER->FLONUM N #b10) can again be used for computing
EXACT->INEXACT on exact integers, which should be 5-15x faster.

There were several places which has a loop to find the bit position of
the most significant bit in a word.  This code has been converted into
a macro, and tweaked to be a bit faster.  Presumably some machines
have this operation as an instruction.