Diode and buffer insertion using the Coloquinte legalizer

[Gabriel Gouvine]

Currently, diode and buffer insertion is quite ad-hoc, and makes use of its own datastructures to decide where free space is available.

This could all be done simply and safely with:

• set the buffer/diode coordinates to a position we'd like (but don't care about overlap)
• run coloquinte::Circuit::legalize

So we'd just have to introduce a new "legalize only" mode to Etesian to support this