Add From impl for TypedArrays

[Pauan]

There's been quite a few issues (1, 2, 3) about TypedArrays and Rust slices.

Right now the only way to convert a Rust slice into a TypedArray is to use the unsafe view function. But it's common for people to make mistakes when using it, and those mistakes are quite bad (since it's unsafe).

So this PR adds in safe From impls for all of the TypedArrays, allowing for easy, safe, and fast conversion from Rust slices.

[alexcrichton]

Seems reasonable to me! Did you want to measure some of the TODO before merging though?

[Pauan]

I ran some benchmarks (sending a 2,560,000 length slice into JS), and the differences are negligible:

Chrome:
  new           309.59499999880790ms  (100%)
  slice len     326.13338709568546ms  (105.34194640191212%)
  slice length  328.74645161322286ms  (106.18597372547494%)
  raw js        330.33241935434840ms  (106.698245848423%)
  slice         331.26806451248063ms  (107.00046191439563%)

Firefox:
  raw js        343.23333333333335ms  (100%)
  slice         344.03333333333336ms  (100.23307780432312%)
  slice len     346.96551724137930ms  (101.08736153827171%)
  slice length  355.10344827586210ms  (103.4583254098343%)
  new           361.75000000000000ms  (105.3947800125618%)

So I went ahead and removed the TODOs.

[alexcrichton]

👍