fix(middleware): types and devtools

[dai-shi]

close #609.

#601 broke typical usage of devtools. And, typing wasn't precise anyway. It's fixed.

There are some refactors too.

[codesandbox-ci]

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Latest deployment of this branch, based on commit ab94691:

Sandbox	Source
React	Configuration
React Typescript	Configuration
React Browserify	Configuration
React Snowpack	Configuration
React Parcel	Configuration
Next.js	Configuration

[github-actions]

Size Change: +21 B (0%)

Total Size: 10.6 kB

Filename	Size	Change
dist/esm/middleware.js	2.51 kB	+10 B (0%)
dist/middleware.js	2.73 kB	+11 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/context.js	622 B
dist/esm/context.js	521 B
dist/esm/index.js	1.2 kB
dist/esm/shallow.js	272 B
dist/esm/vanilla.js	520 B
dist/index.js	1.32 kB
dist/shallow.js	318 B
dist/vanilla.js	630 B

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