Parenthesize unary expressions of prefix operators

[devongovett]

#315 didn't quite fix the issue fully. We also need to handle e.g. prefix update expressions inside a unary expression. ---i is invalid, but -(--i) is valid.

[codecov]

Codecov Report

Merging #426 (2daeda7) into master (6021bf0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   95.44%   95.43%   -0.01%     
==========================================
  Files           1        1              
  Lines        1119     1117       -2     
==========================================
- Hits         1068     1066       -2     
  Misses         51       51              

Impacted Files	Coverage Δ
src/astring.js	95.43% <100.00%> (-0.01%)	⬇️

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[davidbonnet]

Thanks @devongovett for submitting this. 0b273a6 addresses this with less parentheses.