ReadonlySet and ReadonlyMap

[RebeccaStevens]

Fixes #25

Based off #29

[codecov-io]

Codecov Report

Merging #30 into master will decrease coverage by 0.49%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##           master      #30     +/-   ##
=========================================
- Coverage   91.29%   90.79%   -0.5%     
=========================================
  Files          29       28      -1     
  Lines         494      489      -5     
  Branches       95       97      +2     
=========================================
- Hits          451      444      -7     
- Misses         33       34      +1     
- Partials       10       11      +1

Impacted Files	Coverage Δ
src/configs/immutable.ts	0% <ø> (ø)	⬆️
src/configs/all.ts	0% <ø> (ø)	⬆️
src/rules/index.ts	100% <ø> (ø)	⬆️
src/util/typeguard.ts	85.54% <100%> (+0.35%)	⬆️
src/rules/prefer-readonly-types.ts	93.75% <86.66%> (ø)
src/rules/immutable-data.ts	95.12% <0%> (ø)	⬆️
src/util/tree.ts	100% <0%> (ø)	⬆️
src/configs/external-recommended.ts	0% <0%> (ø)	⬆️
src/util/misc.ts	100% <0%> (ø)	⬆️
... and 3 more

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