Expose invalidate API and stop invalidating everything by default

[jackkoenig]

It is safer to error when things are left unconnected. For use cases where various things are expected not to be connected, the user can explicitly invalidate Chisel values (not types).

[ucbjrl]

I've started a Google Doc for the Invalidate API. Please comment or edit that document. Once we've reached an internal consensus, we'll publish the content for public comment.

[colinschmidt]

Presumably this wont affect code that doesn't import chisel3._

[jackkoenig]

This will not affect the compatibility wrapper. It is intended to be improved safety for users of chisel3._

[ucbjrl]

Fixed by #645