Optimized the bn_inverse method.

The new method needs about 30 % less time for prime256k1 and is about
twice as fast for other moduli.  The base algorithm is the same.
The code is also a bit smaller and doesn't need the 8 kb precomputed
table.

Important canges:
1. even/odd distinction so that we need to test only one of the numbers
   for being even.  This also leads to less duplicated code.
2. Allow for shifting by 32 bits at a time in the even test.
3. Pack u,s and v,r into the same array, which saves a bit of stack memory.
4. Don't divide by two after subtraction; this simplifies code.
5. Abort as soon as u,v are equal, instead of subtracting them.
6. Use s instead of r after the loop; no negation needed.
7. New code that divides by 2^k fast without any precomputed values.