Performance issue when handling strings

[FlXME]

Improve performance by using StringBuilder instead of StringBuffer.

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[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[codecov-io]

Codecov Report

Merging #1983 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master   #1983      +/-   ##
===========================================
- Coverage     81.02%     81%   -0.02%     
+ Complexity     6220    6218       -2     
===========================================
  Files           137     137              
  Lines         23130   23130              
  Branches       5356    5356              
===========================================
- Hits          18740   18737       -3     
- Misses         3039    3042       +3     
  Partials       1351    1351

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/alibaba/fastjson/asm/Type.java	77.41% <0%> (ø)	34 <0> (ø)	⬇️
...com/alibaba/fastjson/parser/JSONReaderScanner.java	72.61% <0%> (-1.28%)	44% <0%> (-1%)
...ava/com/alibaba/fastjson/parser/JSONLexerBase.java	71.82% <0%> (-0.04%)	787% <0%> (-1%)

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