Add fromThrowable support for fns returning Promise

[peterboyer]

Currently fromThrowable doesn't play well with functons that return a Promise:

fromThrowable(fnReturnPromise) => Result<Promise<Value>, Error> where we actually want Promise<Result<Value, Error>>

This could be solved as a seperate method, like fromThrowableAsync which accepts only Promise<T> returning functions, or maybe I can infer the return type statically, and then also correctly wrap the given fn to inspect it's return value and wrap if it's a promise accordingly (which would keep the library's API terse and simple).

[peterboyer]

Solved by extending fromThrowable to support Promises, both with TypeScript type inferrence AND checking return type of wrapped function for potential Promise and then wrapping with ok and err respectively.