Output graph files to target directory and improve Graphviz output

[ggiraldez]

The graph files .mmd and .dot files are not committed to the repository and hence writing them to the source directory (inside the generated folder) does not provide any benefit and even has some drawbacks (eg. build errors when switching branches and test are no longer present). This PR changes the output directory to be inside the target directory in the solidity_cargo_tests crate directory.

These output files are opt-in via a RENDER_GRAPHS environment variable. The performance impact of their generation is not too big, but they are generally unnecessary unless we're debugging binding rules.

There are also some tweaks and added information to the .dot file output to improve graph readability and make tracing nodes to rules easier.

[changeset-bot]

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Latest commit: 6b0bf28

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[OmarTawfik]

Left a few suggestions. Thanks!