Additional rounding functions

[calebzulawski]

• to_int (requires comparisons in Comparison functions #36)
• rount_to_int (initially proposed in Rounding and Type Change methods #23)
• trunc (removed in Use platform intrinsics, not LLVM, for floor & ceil #47)
• fract (removed in Use platform intrinsics, not LLVM, for floor & ceil #47)
• round (removed in Use platform intrinsics, not LLVM, for floor & ceil #47)

round_to_int can't be directly implemented in LLVM, since llvm.lround doesn't work on vectors. I'm not sure this function is actually necessary since you can do x.round().to_int().

[workingjubilee]

I am working on the PR that restores these.

[calebzulawski]

Regarding saturating int-to-float rounding:
rust-lang/stdarch#1194 (comment)

The name/semantics of simd_cast are confusing so I'd tend to agree. simd_cast should be safe to call on any vector types of the same lane count, and we should have a new intrinsic that does unsafe int-to-float casts.

[workingjubilee]

Closed by #232.