Repos and Forks can not add multiple domains for the same key

[nhunzaker]

This commit removes the ability to add multiple domains for the same key. This allows us to:

1. Remove the meta-domain
2. Remove the ADD_DOMAIN lifecycle action
3. Avoid merging data to avoid "spilling" keys

Breaking changes

1. Microcosms can no longer implement register(). This was intended to make it easier to build a Microcosm, but it is never used in real-world apps™️ and is problematic for future work.
2. Forks can not add domains on top of parents
3. Microcosms can not add multiple domains for the same key

Fixes https://github.com/vigetlabs/microcosm/issues/427.

[codecov-io]

Codecov Report

Merging #435 into master will decrease coverage by 0.35%. The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
- Coverage   74.37%   74.02%   -0.36%     
==========================================
  Files          45       44       -1     
  Lines        1362     1359       -3     
  Branches      250      253       +3     
==========================================
- Hits         1013     1006       -7     
- Misses        288      293       +5     
+ Partials       61       60       -1

Impacted Files	Coverage Δ
packages/microcosm/src/get-registration.js	100% <ø> (ø)	:arrow_up:
packages/microcosm/src/utils.js	100% <ø> (ø)	:arrow_up:
packages/microcosm/src/addons/indexing.js	0% <0%> (ø)	:arrow_up:
packages/microcosm/src/action.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/microcosm.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/lifecycle.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/domain-engine.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/history.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/addons/action-queue.js	80% <0%> (-20%)	:arrow_down:
packages/microcosm/src/addons/action-button.js	96.87% <0%> (-3.13%)	:arrow_down:

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