- Extended Number `powmod(b, n, m)` to support rational bases `b`.
- Extended Number `powmod(b, n, m)` to support rational bases `b`.

Example:

	say powmod(3/4, 1234, 4171)	#=> 2138

Equivalently:

	say Mod(3/4, 4171)**1234	#=> Mod(2138, 4171)

Negative exponents are also supported:

	say Mod(3/4, 4171)**(-1234)	#=> Mod(2304, 4171)

Defined as:

	powmod(x/y, b, n) = (powmod(x, b, n) * powmod(y, -b, n)) % n

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