Not possible to implement AtLeast32Bit for signed number

[xlc]

Since paritytech/substrate#6367 we have AtLeast32Bit and AtLeast32BitUnsigned so it is obvious that AtLeast32Bit should support signed integer like i128.

However it is currently not possible to use i128 for AtLeast32Bit.

AtLeast32Bit extends BaseArithmetic which extends HasCompact.

HasCompact can only be implemented for unsigned integer (unless someone extend it to make it support signed integer).

Should remove HasCompact from BaseArithmetic and add it to AtLeast32BitUnsigned instead.

[gui1117]

alternatively we could come up with a compact encoding for signed integer maybe paritytech/parity-scale-codec#205