feat/dotgraph: implement dot graph rendering as part of the proc-macro

[drahnr]

The generated files are far from perfect, layout-rs is not working out too well..

The generated files only deviate in the sense that double circles are not rendered accurately, otherwise the output looks fine.

The png rendered lacks text render capabilities, and is hence feature gated for now.

[sandreim]

For Polkadot the output of the graph is quite hard to reason about, so one improvement I was thinking about is to add some parameter such that only selected subsystems are present in the graph. WDYT ?

[drahnr]

@vstakhov @sandreim if you could give it a glance :)

There are trade-offs:

➕ self sufficient svg generation / independence of the environment
➖ slightly increased build time due to additional dependencies

[vstakhov]

Do we really need a png output counting that the conversion svg->png is a trivial thing?

[drahnr]

Do we really need a png output counting that the conversion svg->png is a trivial thing?

Up for debate, happy to delete it

[drahnr]

@vstakhov removed the png generation, (another) review would be much appreciated

[vstakhov]

Rebased the branch