add registers

add registers

Symantic.Parser is now faster than Attoparsec on the Brainfuck benchmarks:

```
$ make benchmarks b=Brainfuck/ByteString/hanoi/'*'

benchmarking Brainfuck/ByteString/hanoi/SymanticParser
time                 11.33 ms   (11.18 ms .. 11.51 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 11.12 ms   (11.03 ms .. 11.20 ms)
std dev              219.9 μs   (164.6 μs .. 301.4 μs)

benchmarking Brainfuck/ByteString/hanoi/Attoparsec
time                 15.38 ms   (15.26 ms .. 15.52 ms)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 15.38 ms   (15.29 ms .. 15.49 ms)
std dev              252.1 μs   (184.3 μs .. 363.2 μs)

benchmarking Brainfuck/ByteString/hanoi/Handrolled
time                 1.608 ms   (1.595 ms .. 1.628 ms)
                     0.999 R²   (0.998 R² .. 1.000 R²)
mean                 1.585 ms   (1.575 ms .. 1.596 ms)
std dev              35.75 μs   (26.53 μs .. 51.66 μs)
variance introduced by outliers: 11% (moderately inflated)
```