Don't use floor for float-to-int conversion

The default rounding for float-to-int conversion should be truncation,
rather than floor.
Use consistent coercion in constructors

Make the SIMD constructors convert their scalar operands to their
desired types.
Simplify constructor coersion

Use |0 instead of a function which does conversion to int32 using a typed
array, and use Math.fround when available for converstion to float32.
Merge pull request #82 from sunfishcode/ctors

Fixes and tests for constructor functions