JSON output + accompanying library

[japaric]

Being able to output JSON, instead of a .dot file, has a few uses:

• it makes our unit tests more reliable: we would be able to use an API instead of text extraction / comparison
• it lets other people perform post-analysis (using cargo xtask for example) relevant to their project. some use cases:
  • does the call graph that stems function X contain a cycle? e.g. does defmt::info-ing a recursive struct requires recursion?
  • can stack usage be upper bounded for this application? use CI to ensure it stays that way
  • is this application free from indirect function calls?
  • RTIC apps contain disconnected components due to the presence of interrupt handlers thus stack cannot be upper bounded for the whole app. with call-stack data + RTIC metadata it should be possible to compute the whole-app stack usage
• plug the data into different visualization format. dot's PNG/SVG output is pretty cluttered and hard to read. using something interactive that limits the number of nodes that are displayed at once (e.g. only show a center node and its immediate neighbors) would make the data easier to digest.

we can probably just serialize a petgraph data structure into JSON to begin with. the node "weight" struct should contain at least this information:

struct Node {
   cumulative_stack_usage: Option<u64>,
   kind: NodeKind,
   local_stack_usage: Option<u64>,
   unmangled_name: String,
}

enum NodeKind {
    // matches to a symbol / routine in machine code
    Concrete,

    // "fictious" node that represents trait object dynamic dispatch
    DynamicDispatch { signature: String },

    // "fictious" node that represents function pointer calls
    FunctionPointer { signature: String },
}