Only try to install the doc directory if it exists.

If you configure with `--disable-docs`, the `doc` directory does not get generated, so
`cp -r doc dist/` fails when you `make dist{,-tar-bins,-doc}` or `make install`
auto merge of #19896 : ktossell/rust/allow-nodoc-install, r=alexcrichton

If you configure with `--disable-docs`, the `doc` directory does not get generated, so the
`cp -r doc dist/` step fails when you `make dist{,-tar-bins,-doc}` or `make install`.
remove TreeMap, TreeSet, TrieMap, TrieSet, LruCache. deprecate EnumSet's std re-export
s/Tree/BTree
enumset fallout
auto merge of #19955 : Gankro/rust/kill-all-code, r=aturon

EnumSet lives on in libcollections so that librustc can still use it. This adds a direct dependency on libcollections to librustc and libserialize.

Should not be merged until https://github.com/rust-lang/rfcs/pull/509 is accepted.

All of these collections have already been moved to collect-rs where they will ideally be maintained and experimented with, or will be replaced by something better: https://github.com/Gankro/collect-rs/

[breaking-change]

r? @aturon @alexcrichton
Remove a ton of public reexports

Remove most of the public reexports mentioned in #19253

These are all leftovers from the enum namespacing transition

In particular:

* src/libstd/io/net/addrinfo.rs
 * SocketType
 * Flag
 * Protocol
* src/libstd/io/process.rs
 * StdioContainer
 * ProcessExit
* src/libstd/num/strconv.rs
 * ExponentFormat
 * SignificantDigits
 * SignFormat
* src/libstd/path/windows.rs
 * PathPrefix
* src/libstd/sys/windows/timer.rs
 * Req
* src/libcollections/str.rs
 * MaybeOwned
* 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/ip.rs
 * IpAddr
* src/libstd/os.rs
 * MemoryMapKind
 * MapOption
 * MapError
* src/libstd/sys/common/net.rs
 * SocketStatus
 * InAddr
* src/libstd/sys/unix/timer.rs
 * Req

[breaking-change]