perf: optimize "msb" by using assembly

[PaulRBerg]

Re-implements #133 on top of the staging branch.

Thanks again for your contribution @t4sk. I made sure that you remain a git author by modifying the git commit body and referencing you there, like this:

Co-authored-by: t4sk <tsk.nakamura@gmail.com>

Also, for the record, I made a few changes to your implementation:

• Updated the function's NatSpec documentation to explain how the new implementation works
• Added comments above each assembly block to give a hint to the user about what power of two factor is being checked
• Renamed f to factor (though please correct me if this stands for something else)
• Swapped the order of the x and result assignment statements in each assembly blocks