"Faster" xsalsa20 and curve25519.

This pull request adds nacl-fast.js, a supposedly faster drop-in replacement for nacl.js.

Turns out curve25519 is not much faster. But xsalsa20 is ~5 times faster. Not sure if we need this.

Very rough benchmarks on my super-slow Atom:

Chrome:
```
BEFORE                                              AFTER

 Benchmarking crypto_stream_xor      Benchmarking crypto_stream_xor
 0.19821605550049554 MB/s            2.3529411764705883 MB/s
                                    
 Benchmarking crypto_onetimeauth     Benchmarking crypto_onetimeauth
 1.697792869269949 MB/s              1.876172607879925 MB/s
                                    
 Benchmarking crypto_secretbox       Benchmarking crypto_secretbox
 0.2014098690835851 MB/s             1.1947431302270013 MB/s
                                    
 Benchmarking crypto_hash            Benchmarking crypto_hash
 395.3488372093023 ops/s             382.3088455772114 ops/s
 0.48053820278712156 MB/s            0.47961630695443647 MB/s
                                    
 Benchmarking secretbox              Benchmarking secretbox
 0.19952114924181963 MB/s            1.1587485515643106 MB/s
                                    
 Benchmarking secretbox.open         Benchmarking secretbox.open
 0.17806267806267806 MB/s            1.016260162601626 MB/s
                                    
 Benchmarking crypto_scalarmult_bas  Benchmarking crypto_scalarmult_bas
 26.515151515151516 ops/s            22.853411687887693 ops/s
                                    
 Benchmarking box                    Benchmarking box
 0.023299161230195712 MB/s           0.02221235006663705 MB/s
                                    
 Benchmarking box.open (valid)       Benchmarking box.open (valid)
 0.023004370830457786 MB/s           0.021958717610891524 MB/s
                                    
 Benchmarking box.open (invalid key  Benchmarking box.open (invalid key
 0.025906735751295335 MB/s           0.02218770800976259 MB/s
```
(note that getting rid of type arrays in core() in favor of normal arrays speeds up salsa in Chrome to ~1 MB/s).

Firefox:

```
 Benchmarking crypto_stream_xor      Benchmarking crypto_stream_xor
 1.017293997965412 MB/s              6.0606060606060606 MB/s
                                    
 Benchmarking crypto_onetimeauth     Benchmarking crypto_onetimeauth
 1.7241379310344827 MB/s             1.9342359767891684 MB/s
                                    
 Benchmarking crypto_secretbox       Benchmarking crypto_secretbox
 0.6269592476489029 MB/s             1.455604075691412 MB/s
                                    
 Benchmarking crypto_hash            Benchmarking crypto_hash
 262.88659793814435 ops/s            252.22551928783383 ops/s
 0.3776435045317221 MB/s             0.40404040404040403 MB/s
                                    
 Benchmarking secretbox              Benchmarking secretbox
 0.6738544474393531 MB/s             1.4124293785310735 MB/s
                                    
 Benchmarking secretbox.open         Benchmarking secretbox.open
 0.5824111822947 MB/s                1.3262599469496021 MB/s
                                    
 Benchmarking crypto_scalarmult_bas  Benchmarking crypto_scalarmult_bas
 16.073478760045923 ops/s            21.315468940316688 ops/s
                                    
 Benchmarking box                    Benchmarking box
 0.015232292460015232 MB/s           0.020479213598197828 MB/s
                                    
 Benchmarking box.open (valid)       Benchmarking box.open (valid)
 0.01525320317266626 MB/s            0.01908761213972132 MB/s
                                    
 Benchmarking box.open (invalid key  Benchmarking box.open (invalid key
 0.016116035455278 MB/s              0.02011263073209976 MB/s
```