Add thunk writes and calls

[zrlk]

It's useful to see deferred calls (e.g. std::thread(f) makes such a call to f) in the call graph. There's a slightly complicated extension to this where:

• write_to_field is a function with semantics (as assigned by semantic/generated and generates) that sets a field f
• call_with_1 calls its function argument with the value 1 (wlog)
• call_with_1(write_to_field) should report write_to_field as a deferred write to f.

We need new edges to represent these deferred actions and support from the various components to emit them.