Add attempt to Either

[jleider]

This adds a convenience function to Either that will produce an Either<Error|E, V> where the left is the result from a try/catch and the right is the result of a fn: () => V.

Since pretty much any type of thing can be thrown in JS the type cant be locked down to just Either<Error, V>. However, adding the union of Error|E will provide at least a little bit of type help to TS users.

Prior art comes from fromTryCatchNonFatal (recently renamed to attempt in v7.3) of ScalaZ disjunctions which are simply right biased Eithers just as in monet.js.

I am happy to change the function name to something like fromTryCatch if that makes it clearer as to what it does.

[ulfryk]

maybe there should be <E extends Error, … constraint? (also Error|E union would be obsolete, just E will do the job)

[ulfryk]

or maybe I'm wrong, as users can throw any custom stuff… but then Error is redundant in union.
Maybe attempt<E = Error, V>(fn: () => V): Either<E, V>; default value will make it more useful!

[ulfryk]

I see only Either<Error | X, … examples here. But it's also important to test how TS handles to infer these type params, and how does it fit for simple types beeing thrown..

[ulfryk]

Additionally - I was thinking about adding Try monad (with a little magic power like the one that Promise has about catching internal exceptions), but for sure explicit .flatMap(v => Either.attempt(throwable(v))) is a much better solution!

[ulfryk]

implemented in #211