Implement new min/max semantics

Following the proposal in
https://github.com/johnmccutchan/ecmascript_simd/issues/67

These min and max functions follow the Math.min and Math.max semantics,
and are arguably the more intuitive and semantics.

This patch also introduces minNum and maxNum functions, which follow the
IEEE-754 minNum and maxNum semantics.
Add tests for minNum and maxNum.
Merge pull request #80 from sunfishcode/minmax

Implement new min/max semantics