Correct span in privacy error
[collections] Adds `drain`: a way to sneak out the elements while clearing.

It is useful to move all the elements out of some collections without
deallocating the underlying buffer. It came up in IRC, and this patch
implements it as `drain`. This has been discussed as part of RFC 509.

r? @Gankro
cc: @frankmcsherry
Add privacy tests
auto merge of #19946 : cgaebel/rust/hashmap-drain-iter, r=gankro

It is useful to move all the elements out of a hashmap without deallocating
the underlying buffer. It came up in IRC, and this patch implements it as
`drain`.

r? @Gankro
cc: @frankmcsherry
auto merge of #19953 : sanxiyn/rust/privacy-span, r=alexcrichton

Fix #19062.
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/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/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]