u160 arithmetic

[aszepieniec]

We need this for block consensus programs, for the difficulty adjustment.

Needed operations:

• comparision (less than)
• addition
• subtraction
• multiplication of a u160 with a u32 (asserting no overflow)
• multiplication of two u160s (asserting no overflow)
• division (not sure we need this one but listed for completeness)

The compiler should be able to handle this as well through an array of 5 u32s.

Co-authored-by: @Sword-Smith