Preserve declarations for private fields, static fields, and methods

[alangpierce]

Fixes #560

Alternate approach to #570 where we emit the declarations as-is, e.g. #foo = 3;
in the class body. Note that this does not transpile these private fields, so
the underlying runtime still needs to support them and will give a syntax error
if it doesn't.

[codecov-io]

Codecov Report

Merging #574 (f8db417) into master (4ff153c) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #574      +/-   ##
==========================================
- Coverage   81.93%   81.86%   -0.07%     
==========================================
  Files          55       55              
  Lines        5800     5569     -231     
  Branches     1311     1313       +2     
==========================================
- Hits         4752     4559     -193     
+ Misses        764      726      -38     
  Partials      284      284              

Impacted Files	Coverage Δ
src/util/getClassInfo.ts	89.14% <100.00%> (+2.53%)	⬆️
src/Options-gen-types.ts	75.00% <0.00%> (-25.00%)	⬇️
src/parser/index.ts	77.77% <0.00%> (-7.94%)	⬇️
src/computeSourceMap.ts	83.33% <0.00%> (-5.56%)	⬇️
src/util/getNonTypeIdentifiers.ts	94.44% <0.00%> (-5.56%)	⬇️
src/parser/plugins/types.ts	91.66% <0.00%> (-3.34%)	⬇️
src/parser/tokenizer/readWord.ts	82.35% <0.00%> (-3.02%)	⬇️
src/parser/traverser/util.ts	86.20% <0.00%> (-2.17%)	⬇️
src/parser/traverser/base.ts	95.45% <0.00%> (-1.69%)	⬇️
src/parser/traverser/statement.ts	82.18% <0.00%> (-0.44%)	⬇️
... and 20 more

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