Refactor motifier implementation to fix various issues

[rrousselGit]

fixes #1534 fixes #1286

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TODO mark maintainState as deprecated in 1.0 TODO deprecate "Reader" in 1.0

[codecov[bot]]

Codecov Report

Merging #1549 (deb8d5d) into master (ea92d3b) will decrease coverage by 4.88%. The diff coverage is 89.56%.

:exclamation: Current head deb8d5d differs from pull request most recent head 8b2d6ca. Consider uploading reports for the commit 8b2d6ca to get more accurate results

@@            Coverage Diff             @@
##           master    #1549      +/-   ##
==========================================
- Coverage   97.61%   92.72%   -4.89%     
==========================================
  Files          34       40       +6     
  Lines        1844     1691     -153     
==========================================
- Hits         1800     1568     -232     
- Misses         44      123      +79     

Impacted Files	Coverage Δ
packages/riverpod/lib/src/devtool.dart	53.57% <ø> (ø)
...ges/riverpod/lib/src/framework/async_selector.dart	96.36% <0.00%> (-3.64%)	:arrow_down:
packages/riverpod/lib/src/framework/selector.dart	92.00% <0.00%> (+0.51%)	:arrow_up:
packages/riverpod/lib/src/synchronous_future.dart	26.31% <26.31%> (ø)
packages/riverpod/lib/src/listenable.dart	61.29% <61.29%> (ø)
packages/riverpod/lib/src/result.dart	68.57% <68.57%> (ø)
...d/lib/src/framework/proxy_provider_listenable.dart	76.66% <76.66%> (ø)
...ter_riverpod/lib/src/change_notifier_provider.dart	87.50% <85.71%> (-12.50%)	:arrow_down:
...ages/riverpod/lib/src/state_notifier_provider.dart	87.50% <85.71%> (-12.50%)	:arrow_down:
packages/riverpod/lib/src/state_provider.dart	85.71% <85.71%> (-14.29%)	:arrow_down:
... and 42 more