Add useService hook and modern Context

[micburks]

RFC

• Implement modern Context API for services
• Implement useService hook
• add tests

[codecov]

Codecov Report

Merging #242 into master will increase coverage by 2.19%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #242      +/-   ##
========================================
+ Coverage    74.8%    77%   +2.19%     
========================================
  Files          13     14       +1     
  Lines         262    287      +25     
  Branches       55     58       +3     
========================================
+ Hits          196    221      +25     
  Misses         49     49              
  Partials       17     17

Impacted Files	Coverage Δ
src/index.js	72.41% <100%> (+0.98%)	⬆️
src/context.js	100% <100%> (ø)

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[ganemone]

Only thing missing is an update to the readme with documentation on the useService hook and might be good to add a test that documents the behavior when you call useService on a token with no value registered. Does it throw an error or return null? I think we probably want it to throw, but not totally sure. Worth some discussion

[micburks]

Only thing missing is an update to the readme with documentation on the useService hook and might be good to add a test that documents the behavior when you call useService on a token with no value registered. Does it throw an error or return null? I think we probably want it to throw, but not totally sure. Worth some discussion

Right now there is no difference between a token that hasn't been registered, a plugin that has no provides, or a plugin that has a provides that returns undefined; they all return undefined. We could throw in all these situations, since it doesn't make sense to use useService if you're not expecting anything to be returned, or we could add another method hasService to the base app in fusion-core so we can have more accurate messaging for errors.

[AlexMSmithCA]

Cool, changes look great!

[micburks]

Needed a mechanism to support Token.optional since flow considers it different from Token. This technically allows someone to pass in something that isn't a token. The usage of this function is so straightforward that I don't think this is a huge issue.

[micburks]

!merge

[old-fusion-bot]

Triggered Fusion.js build verification: https://buildkite.com/uberopensource/fusion-release-verification/builds/1943