rustc: Add --target-cpu flag to select a more specific processor instead of the default, 'generic'.
auto merge of #8444 : erickt/rust/rollup, r=cmr

This merges these PR together:

#8430: r=thestinger 
#8370: r=thestinger
#8386: r=bstrie
#8388: r=thestinger
#8390: r=graydon
#8394: r=graydon
#8402: r=thestinger
#8403: r=catamorphism
std::iterator: Rename .peek() to .inspect()
Add a "peekable" iterator adaptor, with a peek() method that returns the next element.
extra::treemap: Use IteratorUtil::peekable

Replace the previous equivalent iterator adaptor with .peekable().
Refactor the set operation iterators so that they are easier to read.
auto merge of #8404 : stepancheg/rust/zero-unit-inline, r=alexcrichton

Follow-up to #8155
syntax: Shrink enum Token and enum nonterminal

`enum Token` was 192 bytes (64-bit), as pointed out by pnkfelix; the only
bloating variant being `INTERPOLATED(nonterminal)`.

Updating `enum nonterminal` to use ~ where variants included big types,
shrunk size_of(Token) to 32 bytes (64-bit).

I am unsure if the `nt_ident` variant should have an indirection, with
ast::ident being only 16 bytes (64-bit), but without this, enum Token
would be 40 bytes.

A dumb benchmark says that compilation time is unchanged, while peak
memory usage for compiling std.rs is down 3%

Before::

    $ time ./x86_64-unknown-linux-gnu/stage1/bin/rustc --cfg stage1 src/libstd/std.rs
    19.00user 0.39system 0:19.41elapsed 99%CPU (0avgtext+0avgdata 627820maxresident)k
    0inputs+28896outputs (0major+228665minor)pagefaults 0swaps
    $ time ./x86_64-unknown-linux-gnu/stage1/bin/rustc -O --cfg stage1 src/libstd/std.rs
    31.64user 0.34system 0:32.02elapsed 99%CPU (0avgtext+0avgdata 629876maxresident)k
    0inputs+22432outputs (0major+229411minor)pagefaults 0swaps

After::

    $ time ./x86_64-unknown-linux-gnu/stage1/bin/rustc --cfg stage1 src/libstd/std.rs
    19.07user 0.45system 0:19.55elapsed 99%CPU (0avgtext+0avgdata 609384maxresident)k
    0inputs+28896outputs (0major+221997minor)pagefaults 0swaps

    $ time ./x86_64-unknown-linux-gnu/stage1/bin/rustc -O --cfg stage1 src/libstd/std.rs
    31.90user 0.34system 0:32.28elapsed 99%CPU (0avgtext+0avgdata 612080maxresident)k
    0inputs+22432outputs (0major+223726minor)pagefaults 0swaps
add intrinsics for checked overflow add/sub/mul
disable 64-bit CheckedMul on 32-bit

code generation problem reported as issue #8449
vec: optimize the Add implementation

before:

test add ... bench: 164 ns/iter (+/- 1)

after:

test add ... bench: 113 ns/iter (+/- 2)
str: optimize `with_capacity`

before:

test bench_with_capacity ... bench: 104 ns/iter (+/- 4)

after:

test bench_with_capacity ... bench: 56 ns/iter (+/- 1)
move `strdup_uniq` lang item to std::str
fix unused imports
Allow disabling optimizations in tests only
auto merge of #8408 : thestinger/rust/checked, r=Aatch
auto merge of #8412 : thestinger/rust/vector-add, r=alexcrichton
auto merge of #8421 : alexcrichton/rust/unnamed-addr, r=thestinger

This can be applied to statics and it will indicate that LLVM will attempt to
merge the constant in .data with other statics.

I have preliminarily applied this to all of the statics generated by the new
`ifmt!` syntax extension. I compiled a file with 1000 calls to `ifmt!` and a
separate file with 1000 calls to `fmt!` to compare the sizes, and the results
were:

```
fmt           310k
ifmt (before) 529k
ifmt (after)  202k
```

This now means that ifmt! is both faster and smaller than fmt!, yay!
Fixed #8451 - extra::stats::write_boxplot() applied to negative or zero sample values
libsyntax: Update from `@Object` to `@mut Object` as required
librustc: Convert from `@Object` to `@mut Object` as needed