ExprString generates code by ast.Expr like printer.Fprint() 经过拼装，可能与源码不完全一致 TODO 2019.9.28 最开始不知道有 printer 这个包，手写了一坨，后面可能会直接接过去。。 Deprecated.

Find returns a slice of sub-nodes like jQuery.

ReadPackage is simple wrapper for parserDir use pkgName with last segment in pkgPath.

ReadPackageWithName read package with specified package name fileScope used for cache key.

SrcOf print ast.Node to a string.

开了缓存，ops 可以提高三个数量级.