Replace use of YARD::Tags::TypesExplainer with custom parser

[backus]

This parser works decently well but there are some decent bugs (lsegal/yard#1077) that I would like to get past. It is also private API and I it looks like the YARD maintainer has mainly moved on so I probably wont push to improve it and publicize the API

[backus]

Actually it looks like it is this is public API after all. I'll just aim to upstream things

[backus]

Heh actually still unclear whether the actual parse interface is public...