Add a SIMD-enabled version of SideOffsets2D specialized for i32

This will allow for future optimizations in Servo.
Implement is_zero using SIMD and add benchmarks and tests

The SIMD version is approximately comparable in speed to the naive version, but should reduce code size.
Implement add using SIMD and add benchmarks

naive add: 13 ns/iter (+/- 2)
SIMD add: 6 ns/iter (+/- 0)
Inline all SideOffsets2DSimdI32 functions
Merge pull request #31 from bjz/simd

Add a SIMD-enabled version of SideOffsets2D specialized for i32
Merge pull request #33 from mbrubeck/zero

Implement zero() methods for Point2D, Rect, Size2D
Minor style fixes
Remove some unnecessary type constraints