This commit stabilizes the `ErrorKind` enumeration which is consumed by and
generated by the `io::Error` type. The purpose of this type is to serve as a
cross-platform namespace to categorize errors into. Two specific issues are
addressed as part of this stablization:
* The naming of each variant was scrutinized and some were tweaked. An example
is how `FileNotFound` was renamed to simply `NotFound`. These names should not
show either a Unix or Windows bias and the set of names is intended to grow
over time. For now the names will likely largely consist of those errors
generated by the I/O APIs in the standard library.
* The mapping of OS error codes onto kinds has been altered. Coalescing no
longer occurs (multiple error codes become one kind). It is intended that each
OS error code, if bound, corresponds to only one `ErrorKind`. The current set
of error kinds was expanded slightly to include some networking errors.
This commit also adds a `raw_os_error` function which returns an `Option<i32>`
to extract the underlying raw error code from the `Error`.
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