Speed up curve operations

UNROLL ALL THE THINGS! (in nacl-fast)

(Work in progress)

Before:

```
 Benchmarking crypto_scalarmult_base
 169.471 ops/s

 Benchmarking box
 0.158 MB/s
 161.692 ops/s

 Benchmarking box.open
 0.166 MB/s
 170.219 ops/s

 Benchmarking sign
 0.009 MB/s
 73.685 ops/s

 Benchmarking sign.open
 0.004 MB/s
 34.776 ops/s
```

After:

```
 Benchmarking crypto_scalarmult_base
 463.560 ops/s

 Benchmarking box
 0.407 MB/s
 417.293 ops/s

 Benchmarking box.open
 0.596 MB/s
 610.661 ops/s

 Benchmarking sign
 0.022 MB/s
 184.848 ops/s

 Benchmarking sign.open
 0.011 MB/s
 89.146 ops/s
```