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
Make the flag properties use '< 0'.

When interpreting the truth value of an integer in a SIMD value, use the
'< 0' convention.
Clarify comments and code.

More firmly establish the convention of converting from boolean to
integer.
Elementwise select.

This renames the existing bitwise select to bitselect, and introduces a
new elementwise select, following the convention for boolean values in
SIMD of examining the sign bit.
Simplify signMask implementations.