Add writev for files/streams

[rennergade]

We need to add functionality for writev for files and streams. It seems to have broken bash by not supporting it. Should be somewhat straightforward.

Streams:

We should be able to just turn the vector pointers into slices using from_raw_parts and then concatenate them, the we can just call log_to_stdout

File:
We should probably turn the iovecs into IOSlices that way we should be able to pass them to the Rust file API's write_vectored function.