Improved the search by including a levenshtein distance calculation which enables the docs search function to be more forgiving for spelling mistakes
Updated the licensing information for the Levenshtein distance function
Improved the tags impl regex
Sort trait bounds.

Closes #6334
doc: Monty Hall simulation for std::rand

A larger example for std::rand
doc: More efficient Monty Hall simulation
doc: use //! instead of /*! ... */ in std::rand
collections, unicode: Add support for NFC and NFKC
remove unused field CrateContext::uses_gc
Refactored syntax::fold.

Prior to this, the code there had a few issues:

- Default implementations inconsistently either had the prefix `noop_` or
  not.
- Some default methods where implemented in terms of a public noop function
  for user code to call, others where implemented directly on the trait
  and did not allow users of the trait to reuse the code.
- Some of the default implementations where private, and thus not reusable
  for other implementors.
- There where some bugs where default implementations called other default
  implementations directly, rather than to the underlying Folder, with the
  result of some AST nodes never being visited even if the user implemented that
  method. (For example, the current Folder never folded struct fields)

This commit solves this situation somewhat radically by making _all_
`fold_...` functions in the module into Folder methods, and implementing
them all in terms of public `noop_...` functions for other implementors to
call out to.

Some public functions had to be renamed to fit the new system, so this is a
breaking change.

[breaking-change]
Add a few more derivings to AST types
Port Rust to DragonFlyBSD

Not included are two required patches:

* LLVM: segmented stack support for DragonFly [1]

* jemalloc: simple configure patches

[1]: http://reviews.llvm.org/D4705
Fix a whitespace typo
serialize: fix a warning
remove seek from std::io::MemWriter, add SeekableMemWriter to librustc

Not all users of MemWriter need to seek, but having MemWriter
seekable adds between 3-29% in overhead in certain circumstances.
This fixes that performance gap by making a non-seekable MemWriter,
and creating a new SeekableMemWriter for those circumstances when
that functionality is actually needed.

```
test io::mem::test::bench_buf_reader                        ... bench:       682 ns/iter (+/- 85)
test io::mem::test::bench_buf_writer                        ... bench:       580 ns/iter (+/- 57)
test io::mem::test::bench_mem_reader                        ... bench:       793 ns/iter (+/- 99)
test io::mem::test::bench_mem_writer_001_0000               ... bench:        48 ns/iter (+/- 27)
test io::mem::test::bench_mem_writer_001_0010               ... bench:        65 ns/iter (+/- 27) = 153 MB/s
test io::mem::test::bench_mem_writer_001_0100               ... bench:       132 ns/iter (+/- 12) = 757 MB/s
test io::mem::test::bench_mem_writer_001_1000               ... bench:       802 ns/iter (+/- 151) = 1246 MB/s
test io::mem::test::bench_mem_writer_100_0000               ... bench:       481 ns/iter (+/- 28)
test io::mem::test::bench_mem_writer_100_0010               ... bench:      1957 ns/iter (+/- 126) = 510 MB/s
test io::mem::test::bench_mem_writer_100_0100               ... bench:      8222 ns/iter (+/- 434) = 1216 MB/s
test io::mem::test::bench_mem_writer_100_1000               ... bench:     82496 ns/iter (+/- 11191) = 1212 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0000      ... bench:        48 ns/iter (+/- 2)
test io::mem::test::bench_seekable_mem_writer_001_0010      ... bench:        64 ns/iter (+/- 2) = 156 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0100      ... bench:       129 ns/iter (+/- 7) = 775 MB/s
test io::mem::test::bench_seekable_mem_writer_001_1000      ... bench:       801 ns/iter (+/- 159) = 1248 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0000      ... bench:       711 ns/iter (+/- 51)
test io::mem::test::bench_seekable_mem_writer_100_0010      ... bench:      2532 ns/iter (+/- 227) = 394 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0100      ... bench:      8962 ns/iter (+/- 947) = 1115 MB/s
test io::mem::test::bench_seekable_mem_writer_100_1000      ... bench:     85086 ns/iter (+/- 11555) = 1175 MB/s
```

[breaking-change]
std: Make MemWriter clonable
Guide: testing
rustrt: Allow dropping a brand-new Task

When a new task fails to spawn, it triggers a task failure of the spawning task.
This ends up causing runtime aborts today because of the destructor bomb in the
Task structure. The bomb doesn't actually need to go off until *after* the task
has run at least once.

This now prevents a runtime abort when a native thread fails to spawn.
native: Don't deadlock the runtime on spawn failure

Previously, the call to bookkeeping::increment() was never paired with a
decrement when the spawn failed (due to unwinding). This fixes the problem by
returning a "bomb" from increment() which will decrement on drop, and then
moving the bomb into the child task's procedure which will be dropped naturally.
auto merge of #15915 : erickt/rust/master, r=alexcrichton

std: rename MemWriter to SeekableMemWriter, add seekless MemWriter

Not all users of MemWriter need to seek, but having MemWriter seekable adds between 3-29% in overhead in certain circumstances. This fixes that performance gap by making a non-seekable MemWriter, and creating a new SeekableMemWriter for those circumstances when that functionality is actually needed.

```
test io::mem::test::bench_buf_reader                        ... bench:       682 ns/iter (+/- 85)
test io::mem::test::bench_buf_writer                        ... bench:       580 ns/iter (+/- 57)
test io::mem::test::bench_mem_reader                        ... bench:       793 ns/iter (+/- 99)
test io::mem::test::bench_mem_writer_001_0000               ... bench:        48 ns/iter (+/- 27)
test io::mem::test::bench_mem_writer_001_0010               ... bench:        65 ns/iter (+/- 27) = 153 MB/s
test io::mem::test::bench_mem_writer_001_0100               ... bench:       132 ns/iter (+/- 12) = 757 MB/s
test io::mem::test::bench_mem_writer_001_1000               ... bench:       802 ns/iter (+/- 151) = 1246 MB/s
test io::mem::test::bench_mem_writer_100_0000               ... bench:       481 ns/iter (+/- 28)
test io::mem::test::bench_mem_writer_100_0010               ... bench:      1957 ns/iter (+/- 126) = 510 MB/s
test io::mem::test::bench_mem_writer_100_0100               ... bench:      8222 ns/iter (+/- 434) = 1216 MB/s
test io::mem::test::bench_mem_writer_100_1000               ... bench:     82496 ns/iter (+/- 11191) = 1212 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0000      ... bench:        48 ns/iter (+/- 2)
test io::mem::test::bench_seekable_mem_writer_001_0010      ... bench:        64 ns/iter (+/- 2) = 156 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0100      ... bench:       129 ns/iter (+/- 7) = 775 MB/s
test io::mem::test::bench_seekable_mem_writer_001_1000      ... bench:       801 ns/iter (+/- 159) = 1248 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0000      ... bench:       711 ns/iter (+/- 51)
test io::mem::test::bench_seekable_mem_writer_100_0010      ... bench:      2532 ns/iter (+/- 227) = 394 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0100      ... bench:      8962 ns/iter (+/- 947) = 1115 MB/s
test io::mem::test::bench_seekable_mem_writer_100_1000      ... bench:     85086 ns/iter (+/- 11555) = 1175 MB/s
```