Symbols, Promise, and API cleanup

See the change notes in the readme: https://github.com/uniphil/results#v070

Results is getting closer to a stable release, despite the number of breaking changes in this one. A lot of the breakage this time is from cleaning up parts of the API that don't really make sense, and weren't useful -- the useful parts should mostly be compatible.

The next release might have one major breaking change depending on how #38 plays out, but otherwise it should be mostly non-breaking features and performance from here on.