std::trie: add benchmarks for insert.
std::trie: optimise insert slightly.

This reduces the number of moves/memcpy's we do, which makes insert
faster, especially in cases of keys with long equal prefixes (the
_low_bits tests):

Before:

    bench_insert_large                ... bench:    553966 ns/iter (+/- 64050)
    bench_insert_large_low_bits       ... bench:   1048151 ns/iter (+/- 92484)
    bench_insert_small                ... bench:    168840 ns/iter (+/- 22410)
    bench_insert_small_low_bits       ... bench:    185069 ns/iter (+/- 38332)

After:

    bench_insert_large                ... bench:    422132 ns/iter (+/- 35112)
    bench_insert_large_low_bits       ... bench:    339083 ns/iter (+/- 34421)
    bench_insert_small                ... bench:    134539 ns/iter (+/- 15254)
    bench_insert_small_low_bits       ... bench:     88775 ns/iter (+/- 5746)
Removed redundant code, improve performance

closes #10535
auto merge of #11546 : huonw/rust/trie-insert, r=alexcrichton

This reduces the number of moves/memcpy's we do, which makes insert
faster, especially in cases of keys with long equal prefixes (the
\_low_bits tests):

Before:

    bench_insert_large                ... bench:    553966 ns/iter (+/- 64050)
    bench_insert_large_low_bits       ... bench:   1048151 ns/iter (+/- 92484)
    bench_insert_small                ... bench:    168840 ns/iter (+/- 22410)
    bench_insert_small_low_bits       ... bench:    185069 ns/iter (+/- 38332)

After:

    bench_insert_large                ... bench:    422132 ns/iter (+/- 35112)
    bench_insert_large_low_bits       ... bench:    339083 ns/iter (+/- 34421)
    bench_insert_small                ... bench:    134539 ns/iter (+/- 15254)
    bench_insert_small_low_bits       ... bench:     88775 ns/iter (+/- 5746)

Notably: no unsafe code.
remove reference counting headers from ~

Unique pointers and vectors currently contain a reference counting
header when containing a managed pointer.

This `{ ref_count, type_desc, prev, next }` header is not necessary and
not a sensible foundation for tracing. It adds needless complexity to
library code and is responsible for breakage in places where the branch
has been left out.

The `borrow_offset` field can now be removed from `TyDesc` along with
the associated handling in the compiler.

Closes #9510
Closes #11533
remove `borrow_offset` as ~ is now free of headers
add implementation of `Repr` for `~[T]`
auto merge of #11547 : b1nd/rust/rust-doc, r=alexcrichton

closes #10535
auto merge of #11535 : thestinger/rust/header, r=alexcrichton

Unique pointers and vectors currently contain a reference counting
header when containing a managed pointer.

This `{ ref_count, type_desc, prev, next }` header is not necessary and
not a sensible foundation for tracing. It adds needless complexity to
library code and is responsible for breakage in places where the branch
 has been left out.

The `borrow_offset` field can now be removed from `TyDesc` along with
the associated handling in the compiler.

Closes #9510
Closes #11533