Changed Get64(.) to GetLow64()

The function Get64(.) has a bug in case the width is not divisible by 64.
Since it is only ever used as Get64(0) this simply changes it to this
special case.
Unittests for uint256.h

Unit tests for uint256.h. The file uint160_tests.cpp is no longer
needed. The ad-hoc tests which were in uint256.h are also no longer
needed. The new tests achieve 100% coverage.