Add S-expression parser

[johnynek]

Relates to #282 and #628

[codecov-io]

Codecov Report

Merging #645 (d0faae8) into master (0ef6918) will increase coverage by 0.08%. The diff coverage is 98.38%.

@@            Coverage Diff             @@
##           master     #645      +/-   ##
==========================================
+ Coverage   91.87%   91.95%   +0.08%     
==========================================
  Files          82       83       +1     
  Lines        7407     7456      +49     
  Branches      328      329       +1     
==========================================
+ Hits         6805     6856      +51     
+ Misses        602      600       -2     

Impacted Files	Coverage Δ
core/src/main/scala/org/bykn/bosatsu/SExpr.scala	98.00% <98.00%> (ø)
core/src/main/scala/org/bykn/bosatsu/Json.scala	94.36% <100.00%> (ø)
...e/src/main/scala/org/bykn/bosatsu/StringUtil.scala	98.63% <100.00%> (+1.33%)	:arrow_up:
core/src/main/scala/org/bykn/bosatsu/Value.scala	94.59% <0.00%> (-0.91%)	:arrow_down:
.../src/main/scala/org/bykn/bosatsu/Declaration.scala	98.89% <0.00%> (+0.22%)	:arrow_up:
...n/scala/org/bykn/bosatsu/codegen/python/Code.scala	95.39% <0.00%> (+0.46%)	:arrow_up:
.../src/main/scala/org/bykn/bosatsu/ValueToJson.scala	88.39% <0.00%> (+0.55%)	:arrow_up:

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