refact(jsdoc/jsdocFileReader): use estraverse rather than hacked walk.js

The estraverse project is a well maintained and comprehensive library for
traversing the AST generated by esprima. We were using a file taken from
the Acorn parser but this had no tests and was modified from the original.

**BREAKING CHANGE**: the AST node attached by this fileReader to docs was
under the `doc.codeNode.node` property. This is now simply `code.codeNode`.
The `codeNameProcessor` has been updated accordingly but if you had written
your own processor that used `doc.codeNode` then you should update it
before updating past this change.