Use consistent bases in the constant powers of two

[PaulRBerg]

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In the PRBMath.sol contract, the msb function defines the powers of two as 2**n (in base 10), whereas sqrt defines them using hexadecimal notation 0x10... (base 16).

It would be nice if we used the same base for all constant powers of two used in this contract.

This would have made it easier to spot the error explained in https://github.com/paulrberg/prb-math/issues/97.