libs: clean up enums now that namespacing has arrived

[aturon]

Now that enums are namespaced, there's a fair amount of cleanup work to do in libstd.

In particular:

• During the rollout of the feature, variants were re-exported at the top level to minimize churn. But the new idiom is to keep these items namespaced under the enum. In most cases, these re-exports should be removed.
• Some variants included prefixes or suffixes in their names as a way to work around the lack of namespacing. These should be renamed.

A lot of this work will happen naturally during API stabilization, but it's also something that can tackled in parallel to help speed things up.

[aturon]

cc @sfackler @japaric

[aturon]

Does someone want to make a list of the various enums this should apply to, so we can triage?

[fhartwig]

Here's a list of enums whose variants are re-exported, by file:

• src/libstd/collections/hash/map.rs: Entry
• src/libstd/collections/hash/table.rs: BucketState
• src/libstd/dynamic_lib.rs: Rtld
• src/libstd/io/mod.rs: SeekStyle, FileMode, FileAccess, FileType, IoErrorKind
• src/libstd/io/net/addrinfo.rs: SocketType, Flag, Protocol
• src/libstd/io/net/ip.rs: IpAddr
• src/libstd/io/process.rs: StdioContainer, ProcessExit
• src/libstd/num/strconv.rs: ExponentFormat, SignificantDigits, SignFormat
• src/libstd/os.rs: MemoryMapKind, MapOption, MapError
• src/libstd/path/windows.rs: PathPrefix
• src/libstd/sys/common/net.rs: SocketStatus, InAddr
• src/libstd/sys/unix/timer.rs: Req
• src/libstd/sys/windows/timer.rs: Req
• src/libcore/cmp.rs Ordering
• src/libcore/result.rs: Result
• src/libcore/option.rs: Option
• src/libcollections/str.rs: MaybeOwned

I hope I haven't missed any.
Apart from Result, Option and (maybe) Ordering, I don't think any of these need to have their variants re-exported.

[sfackler]

We don't really need to reexport Result's, Option's and Ordering's either since everyone will be pulling them from the prelude anyway.