Crypto: update checked addition functions to use CheckedAdd intrinsic.

The shift_add_check_overflow and shift_add_check_overflow_tuple functions are
re-written to be more efficient and to make use of the CheckedAdd instrinsic
instead of manually checking for integer overflow.

* The invokation leading_zeros() is removed and replaced with simple integer
  comparison. The leading_zeros() method results in a ctpop LLVM instruction
  and it may not be efficient on all architectures; integer comparisons,
  however, are efficient on just about any architecture.
* The methods lose the ability for the caller to specify a particular shift
  value - that functionality wasn't being used and removing it allows for the
  code to be simplified.
* Finally, the methods are renamed to add_bytes_to_bits and
  add_bytes_to_bits_tuple to reflect their very specific purposes.
Crypto: Add tests for add_bytes_to_bits functions.