Improve Flow coverage

[alxmyth]

#### Problem/Rationale

Fusion.js packages, with the exception of `fusion-core` do not export [libdef](https://flow.org/en/docs/libdefs/) and consumers rely on the Flow type definitions embedded in the source code.  Given this, it is a pain point for consumers when type definitions are missing or incomplete.

We should also strive for full Flow coverage to minimize issues within our own packages.

#### Solution/Change/Deliverable

Two-fold:
* Reach 100% Flow coverage on exported type definitions.
* Strive for 100% Flow coverage internally for each package.

(compare to before)

[codecov]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #105   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files           2        2           
  Lines          27       27           
  Branches        4        4           
=======================================
  Hits           26       26           
  Partials        1        1

Impacted Files	Coverage Δ
src/plugin.js	100% <ø> (ø)	⬆️
src/hoc.js	94.44% <100%> (ø)	⬆️

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[KevinGrandon]

Is using * any better in these cases?

[alxmyth]

In theory, I think it would be. Since we don't necessarily care what the input is as long as it matches the needs of withRPCRedux (a la, go ahead and infer that type and enforce whatever you infer). It does look like the existential type may soon fall out of favor as of 0.0.72 though.

In either case, coverage won't change here. The inferred type will be any as that's what fusion-rpc-redux currently expects.