Parse Atoms which start with a letter or symbol

`<|>` is a Parser combinator, the choice operator. It will try the first
parser, if it doesn't match anything, try the second parser, and so on.

`a:b` creates a list from `a` and `b`. Another possibility would be to
use `[a] ++ b`, where a new list with a single element is created `[a]`
and concatenated with `++`