Add xdot support and out-of-process layout computations

[chtenb]

• Fixes Change string marshaling to something crossplatform #55
• Fixes Change dot rendering to be done out-of-process #56
• Makes parallel layout computations possible
• Adds xdot layout information, which is richer than what we currently expose
• Makes the API less error prone; no more footguns w.r.t. calling methods in the right order, risking crashing the application when not done correctly, etc
• Consume dot algorithm in a more battle tested way, by calling dot.exe, instead of consuming as a library. Experience has shown that consuming graphviz as a library is prone to uprooting lingering issues that are not triggered when calling dot.exe. Example: Generation bug #50
• Fixes Fix flaky test TestRecordShapeAlignment #30 by applying a workaround using the new xdot information
• Fixes Splines() throws error when edge has tail arrows enabled #61
• Fixes issues with exporting and importing unnamed edges from dot by disallowing empty strings in creation and lookup methods.
• Make SetAttribute methods safe by default, by introducing non-existing attributes with the empty string as default value.
• Extend and streamline layout information methods