Guard functions requiring external graphviz binary behind "graphviz-exec" feature flag

[asglover]

This issue and associated PR would make egraph-serialize follow the conventions in graphviz-rust regarding relying on the external graphviz CLI tool.

Graphviz-rust guards functions which require the existence of the external graphviz CLI binary behind a feature flag "graphviz-exec". This allows users who just want to create dot files and only use the rust native graphviz-rust features to do so with confidence that they aren't relying on the existence of the CLI tool.

https://github.com/besok/graphviz-rust/blob/de7946f2cb84292580579d17f53514b145c53f8a/src/lib.rs#L115-L116

I drafted a PR which would adopts this convention for egraph-serialize.

#24

The PR:

• Adds the feature flag graphviz-exec
• Puts to_svg_file behind this flag
• Adds documentation about the new flag
• Modifies the testing so that the testing is not based on a runtime test for the existence of the CLI tool, but instead on the feature flag.
• Modifies the testing so that tests which use serde gated features only run when serde is available.

The CI only runs with all feature available, but I didn't want to mess with CI pipeline before having a checking in with how you feel about this PR / issue generally. @saulshanabrook Do you have some feedback about this proposal?

The motivation is that we want to be able to use graphviz-rust without the CLI tool for our base functionality.
And guard anything which requires the CLI tool behind a feature flag.

[saulshanabrook]

Thanks @asglover! That all makes sense to me and the PR looks good.

[asglover]

Thanks! I moved the PR from Draft to Regular PR based on this feedback! Happy Holidays!