tree shake

[sparklinm]

ES6 模块 tree shake 的实现，同时这里也尝试进行了 commonjs 的 tree shake。一个完整的 tree shake 过程分为以下两步：

1. 跨文件分析导出模块是否被导入（已经实现）。
2. 分析未被导入的导出模块是否在本文件内部使用（简单实现），但可以借助 terser 工具处理。

（类似于webpack 的 usedExports）

[codecov-commenter]

Codecov Report

Merging #174 (95c2cbd) into master (c981924) will not change coverage.
The diff coverage is n/a.

❗ Current head 95c2cbd differs from pull request most recent head 29e2ef1. Consider uploading reports for the commit 29e2ef1 to get more accurate results

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files          65       65           
  Lines        1390     1390           
  Branches      385      385           
=======================================
  Hits         1255     1255           
  Misses        112      112           
  Partials       23       23           

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[Genuifx]

ref #212