Memory safety in vector

Vector export a lot of functions which allows user to violate memory safety. I think it should be very straightforward to distinguish between memory-safe and memory-unsafe functions. Currently this is not the case: #118 as example of seemingly benign functions. #188 is such example as well. `unstream` implementation blindly trusts programmer supplied length and happily overwrites buffer.

I think good way to mark unsafe functions as unsafe is to either use `unsafe` prefix (unsafeIndex/unsafeWrite/...) or export from Unsafe/Internal module. This will be a lot of work but it on the plus side it will require handling memory safety in systematic way instead of playing fast and loose