Add SIMD accelerated multiple pattern search.

[BurntSushi]

This uses the "Teddy" algorithm, as learned from the Hyperscan regular
expression library.

This support is optional, subject to the following:

1. A nightly compiler.
2. Enabling the simd-accel feature.
3. Adding RUSTFLAGS="-C target-feature=+ssse3" when compiling.

[BurntSushi]

Note that this PR is blocked on a new release of simd making its way to crates.io. :-)

cc @huonw @alexcrichton

@killercup I may be able to carve out some interesting projects worth mentoring from this. I left quite a number of TODOs. Feel like learning SIMD? :-)

[BurntSushi]

Relevant benchmarks:

name                                   rust.master ns/iter   rust.simd ns/iter       diff ns/iter   diff %
sherlock::name_alt3                    1,153,246 (515 MB/s)  187,304 (3,176 MB/s)        -965,942  -83.76%
sherlock::name_alt4_nocase             1,223,618 (486 MB/s)  293,523 (2,026 MB/s)        -930,095  -76.01%
sherlock::name_alt5                    319,736 (1,860 MB/s)  182,599 (3,258 MB/s)        -137,137  -42.89%
sherlock::name_alt5_nocase             1,223,311 (486 MB/s)  726,282 (819 MB/s)          -497,029  -40.63%
sherlock::name_holmes_nocase           1,108,772 (536 MB/s)  258,606 (2,300 MB/s)        -850,166  -76.68%
sherlock::name_sherlock_holmes_nocase  1,159,518 (513 MB/s)  239,155 (2,487 MB/s)        -920,363  -79.37%
sherlock::name_sherlock_nocase         1,160,342 (512 MB/s)  235,768 (2,523 MB/s)        -924,574  -79.68%
sherlock::the_nocase                   1,643,616 (361 MB/s)  461,669 (1,288 MB/s)      -1,181,947  -71.91%

[killercup]

Cool! Thank you for thinking of me! I'll have a look at this :)

[killercup]

Fiddling with this a bit I noticed (aside from the stuff I did in #232) that there seems to be a genuine underflow exposed by the fowler::match_basic_81 test case: In src/simd_accel/teddy128.rs:573, you call verify_128 with pos - 2. But pos is usize and doesn't appear to be strictly > 2 here.

Edit: I added a simple check to prevent the underflow in #232.

[BurntSushi]

@killercup Thanks! I've fixed that in this PR. (I didn't see your edit.)

[BurntSushi]

@llogiq FYI, this PR impacts the regex-dna benchmark. It makes the multithreaded version slightly faster (0.69s down to 0.63s on my system), but it makes the single threaded version twice as fast (2.55s down to 1.23s). We'll have to wait for simd on stable to get this into the benchmark game though!