Add entity parsing

[andresmanelli]

Hello, I opened this PR in the hdlparse project.

This is a follow-up to include entity symbol generation.

I'm no python developer so I'm not sure if the code is great, but it works quite well for me.

Could you take a look to both PRs and tell be if there's anything I can do to make them merge-ready?

Thanks !

[Paebbels]

See https://github.com/hdl/symbolator as a fork.