Improve TypedArray::to_vec performance by not zero-initializing buffer

[adrian17]

You can see the rationale and results in #2639 .

Changes:

• changed raw_copy_to to take pointers instead of slices (as slices require data to be pre-initialized)
• made it unsafe, as even though technically we don't use any unsafe APIs, invalid use can modify random memory
• made it pub (this part is optional, let me know if you don't want this PR to extend public API)
• use it in to_vec(). The general pattern of with_capacity->modify through pointer->set_len is similar to the official example in https://doc.rust-lang.org/std/vec/struct.Vec.html#method.set_len , and looks sound in general. Still, this does add a significant amount of unsafe uses.

[alexcrichton]

Seems reasonable to me, thanks!