A nom-based ABNF parser.

Not thoroughly tested, but "works for me" (and with arguably complex ABNFs)

There are some experimental branches. The with_generic_node branch uses an enum...

enum Node {
    Alternation(Vec<Node>>),
    Concatenation(Vec<Node>>),
    ...
}

...which may be more suitable when implementing transformations.

The master branch is a direct transformation of RFC 5234 to code.