ocr-numbers: Improve code based on HLint's suggestions.
ocr-numbers: Improve code based on HLint's suggestions.

In this case, we had a suggestion...

```
./src/Example.hs:20:25: Suggestion: Redundant $
Found:
  flip zip ['0' .. '9'] $ [ ...huge list... ]
Why not:
  flip zip ['0' .. '9'] [ ...huge list... ]

1 hint
```

... that lead to a warning:

```
./src/Example.hs:20:25: Warning: Redundant flip
Found:
  flip zip ['0' .. '9'] [[ ...huge list... ]]
Why not:
  zip [[ ...huge list... ]] ['0' .. '9']

1 hint
```

This is undesirable, because it would be difficult to read the
code with a big list in the middle, so we moved the list to a
new variable, and we also took the opportunity to use an infinite
list:

```
zip ocrDigits ['0'..]
```

exercism/xhaskell