Change SerialPort to a struct, and move the implementation to a "sys"
module which selects between different possible "real" implementations.

Add a public posix module containing a posix-specific SerialPortExt
trait and the BreakDuration type.

Get the windows build working. Remove the windows-specific extensions
module because windows doesn't have any methods not available on posix.

One of the CI toolchains uses an older version of rustc, which doesn't
include as_deref, so we just do the same thing that as_deref does
internally.

Implementing the extension in the posix extension mod ensures it will
appear in documentation even if documentation is built on windows, since
that mod is cfg(doc).

Current timeouts are based on
[this comment from !78](https://gitlab.com/susurrus/serialport-rs/-/merge_requests/78#note_343695538),
copying the windows timeout settings needed to get posix-like timeout
behavior. That is, reads should return available data ASAP regardless of
timeouts, but stop early when there is a timeout.

EventCache will hold the HANDLE for a read/write when not in use or will
produce a new handle for each thread when multiple threads try to
read/write at the same time. The cache holds a single value and will
deallocate extra handles when there is already a handle stored. We have
one cache for a read_handle and one cache for a write_handle.

The expectation is that in the normal case, at most one thread will be
reading/writing at a time, and so at most one handle will be created for
each of read and write. But in abnormal cases, when multiple threads try
to read/write at the same time, we auto-create and auto-close extra
handles as needed.

Fix the into_raw_handle/into_raw_fd to drop other fields that were
previously leaked.