Merge #2772

2772: snocket: introduce LocalSocket and FileDescriptor newtypes r=coot a=coot

Provide a `LocalSocket` newtype wrapper and `FileDescriptor` type (newtype wrapper too).

`LocalSocket` is a uniform newtype for `HANDLE` on `Windows` and `Socket` on `Posix`.

FileDescriptor is a newtype wrapper for file descriptor numbers.  For
sockets or unix sockets we use file descriptor numbers, for windows
named pipes (`HANDLE`) we use the memory address as the file descriptor number. 
`FileDescriptor` only exposes `Eq` and `Show` instances, the constructor is private.


Co-authored-by: Marcin Szamotulski <[email protected]>

Author: iohk-bors[bot]