speedup and improve Travis builds

OPAM now has a binary distribution, so use that. Speedup might result
from setting OPAMJOBS since Travis build machines have 2 cores.
Avoiding installation of OCaml system compiler won’t help because we
now have to install one at opam-init time instead.

Also simplify pinning. No longer need to add future-dev repo due to new
opam features.
rename module StrandName to Strand

It’s simpler, and StrandName’s camel casing wasn’t following our module
naming convention of using underscores.
Strand: improve API

* Make type be a private char.
* Use new convention of being exception-less.
* Provide constants: minus and plus.
* of_string now accepts more strings.
* Remove compare and equal functions.
* minus_plus now return a char instead of a string.