Improve Flow type definitions for exported functions

[alxmyth]

Fixes #685

#### 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)

[alxmyth]

Given the lower test coverage here, I opted to type the exports with the types identified in README.md. Some of these may need to change to meet edge cases, and so I avoided investing time in typing this entire package.

[codecov]

Codecov Report

Merging #93 into master will increase coverage by 0.62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage    77.3%   77.93%   +0.62%     
==========================================
  Files           8        9       +1     
  Lines         141      145       +4     
  Branches       36       36              
==========================================
+ Hits          109      113       +4     
  Misses         21       21              
  Partials       11       11

Impacted Files	Coverage Δ
src/middleware.js	20% <100%> (+20%)	⬆️
src/index.js	100% <100%> (ø)
src/dispatched.js	12.5% <0%> (-12.5%)	⬇️

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[alxmyth]

Most of the relevant changes take place in this file.

[KevinGrandon]

Interesting approach! Happy to try this for now.