net: add {Tcp, Unix}Stream::try_{read, write}_vectored

[Kestrer]

Motivation

try_read and try_write on these two types already exist, so it makes sense to add their vectored I/O counterparts. I needed this when implementing vectored writes on a shared TCP stream type (which makes use of writable/try_write instead of the AsyncWrite implementation).

Solution

Add these methods.

[Darksonn]

This seems good to me.