Fix sb-gmp:mpz-pow for non-bignum bases

Don't declare types, and we don't need the check-type either
Fix for sb-gmp bignum result allocation (lp#1206191)

Patch based on 59e72c69c00abf9095d82e9a7b65f9c1b54d105c from
<https://github.com/sfrank/sb-gmp> (thanks to Stephan Frank)
Save some space in WITH-OUTPUT-TO-STRING.

Not passing :element-type 'character to MAKE-STRING-OUTPUT-STREAM
saves a few bytes, since it's the default value.