Add note about string indexing.

Thanks @chris-morgan!
Revamp doc index

Fixes #14972
Relax lifetime bounds on Reader/Writer impls for trait boxes

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>`
for 'a other than 'static.
Add unwrap method to MoveItems
Fix whitespace and missing parentheses.
Have std::io::TempDir::new and new_in return IoResult

This allows using `try!()`

[breaking-change]

Fixes #16875
std: branchless bucket distance for hashmap
std: RawTable exposes a safe interface for HashMap

Introduced a new growth algorithm.
std: Split hashmap.rs into modules
native: Remove a bogus assert in net::read

This assert was likely inherited from some point, but it's not quite valid as a
no-timeout read may enter this loop, but data could be stolen by any other read
after the socket is deemed readable.

I saw this fail in a recent bors run where the assertion was tripped.
Fix spelling errors and capitalization.
Allow trailing commas in bitflags! macro
Test attributes
Attach documentation to the bitflags! macro itself

This is in consistent with the style demonstrated in the std::macros module.
Use {} for bitflags! definition and invocations

This looks nicer because it reflects Rust's other syntactic structures.
auto merge of #16982 : jbcrail/rust/comment-and-string-corrections, r=alexcrichton

I corrected spelling and capitalization errors in comments and strings.
Auto-derive Clone for syntax::diagnostic::Level.
auto merge of #16986 : bjz/rust/bitflags, r=alexcrichton

Closes #16469
std: Refine and document HashMap's code

* branchless `bucket.next()`
* robin_hood is a free function
* fixed the resize policy that was off by one
* documented the growth algorithm
* updated documentation after interface changes
* removed old fixmes
std: Fix overflow of HashMap's capacity