add initial `Iterator` support
add crate attributes for `rustdoc`
add support for detached threads
use a separate shim for detached threads
privatize some needlessly public methods
remove the `RcMut` type

This will be replaced by simply using `Rc` with a dynamically mutable
inner type.
implement `Eq` and `Ord` for `Rc`
add implementations of `Eq` and `Ord`
implement `Eq` and `Ord` for `Arc`
add `replace` function
add min and max functions
add missing `Eq` and `Ord` implementations
add an initial `HashMap`
add a `ConcurrentHashMap` type
add a `ShardMap` wrapper around `ConcurrentHashMap`
implement `Hash` for ()
add a test for SipHash
ShardMap: remove the redundant `shards` field
concurrent: make the maps proper shared containers
concurrent: add docs and a missing `Clone` impl