Refactor context usage

[micburks]

Provide way to opt-out of legacy context for existing plugin HOCs. This will provide a path forward for existing plugins that use ProviderPlugin/HOC to consume services.

Changes

• Refactor withServices to use ServiceContext rather than creating a new Context
• Remove React.createContext ponyfill from withServices - only use new context
• Add hoc.create argument to opt out of legacy context, using ServiceContext instead

Tests

• Update test descriptions
• Tests for withServices
• Tests for HOC

Result
By passing the token into hoc.create, an HOC can opt out of using legacy context. You can now replace

app.register(MyToken, ProviderPlugin.create('name', MyPlugin));

with

app.register(MyToken, MyPlugin);

and your component tree will thank you.

[codecov]

Codecov Report

Merging #245 into master will increase coverage by 1.8%.
The diff coverage is 92.5%.

@@            Coverage Diff            @@
##           master     #245     +/-   ##
=========================================
+ Coverage      77%   78.81%   +1.8%     
=========================================
  Files          14       13      -1     
  Lines         287      269     -18     
  Branches       58       57      -1     
=========================================
- Hits          221      212      -9     
+ Misses         49       40      -9     
  Partials       17       17

Impacted Files	Coverage Δ
src/index.js	71.42% <ø> (-0.99%)	⬇️
src/hoc.js	92.3% <91.3%> (-1.45%)	⬇️
src/context.js	97.5% <94.11%> (-2.5%)	⬇️

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[micburks]

The diff on the README is a mess. Summary:

• Add [DEPRECATED] to ProviderPlugin, Provider, ProvidedHOC
• Moved new methods to the top
• Moved deprecated methods to the bottom
• middleware link was dead, removed
• Added API reference for withServices, mysteriously missing
• Document new token parameter for ProvidedHOC.create
• Replace PluginProvider/HOC example with useService example

[micburks]

!merge

[fusionjs-bot]

Triggered Fusion.js build verification: https://buildkite.com/uberopensource/fusion-release-verification/builds/2005