port branch combinators to the new design

[Geal]

I got started on a function version of alt in src/branch/mod.rs. The combinators to rewrite are in src/branch/macros.rs. To rewrite:

• alt
• alt_complete (maybe remove this one?)
• switch
• permutation: that's a big one, but it might be easier to rewrite as a function

[Geal]

alt is now reimplemented here: https://github.com/Geal/nom/blob/master/src/branch/mod.rs#L89-L93
it works by accepting a tuple of parsers as argument.
I'm not sure switch should really be reimplemented, since it can be written directly as a pattern match.
permutation will be tricky to do

[Geal]

after some tests, it appears switch! can easily be replaced with a closure that does pattern matching before calling other parsers. permutation is still not done

[Geal]

permutation was done in 66dec05

[Geal]

alt, switch and permutation now have function equivalent, but their underlying implementation will not use them