Add FutureResult to return a Future that immediately resolves

[garious]

Fixes #141

[garious]

Thanks @rlkelly, but I wanted your contribution now. :)

Don't worry about those return values. I see that it's a pain to handle those alongside futures, and I'm planning to rewrite a lot of these functions anyway. I'll take care of it over that.

[codecov-io]

Codecov Report

Merging #150 into master will decrease coverage by 0.04%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   96.47%   96.43%   -0.05%     
==========================================
  Files          19       19              
  Lines        2072     2076       +4     
==========================================
+ Hits         1999     2002       +3     
- Misses         73       74       +1

Impacted Files	Coverage Δ
src/lib.rs	100% <ø> (ø)	⬆️
src/accountant_skel.rs	95.57% <100%> (ø)	⬆️
src/accountant_stub.rs	92.95% <81.25%> (-1.08%)	⬇️

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[rlkelly]

should we make a ticket for the error handling you mentioned in your comments?

[garious]

yep, will do