Fastjson fixes enhancements

[arjun12102019]

What this PR does / why we need it?

Optimized code by using Java streams instead of using former for loops to improve performance when we create SymbolTable object by passing big array of elements.
Written basic Junits for SymbolTable class

Summary of your change

Used Java streams to optimize the code as streams improve the performance.
Added Junits for SymbolTable class.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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[kraity]

Hi, 这个优化优化一般适用于业务层, 虽然增强了阅读性, 但是实际运行性能会降低.