fix problems with single-flonum (versus flonum) results

The problems were with

 * `sqrt' and `expt' on single-flonum complex numbers
 * `asin' and `acos' on single-flonum arguments and complex results
 * `atan' on mixtures of single-flonum and exact arguments
 * `gcd' on mixtures of single-flonum and flonum arguments
math/matrix: fix some duplicate documentation tags