Add support for &'static data to the interpreter

[willcrichton]

Currently, the interpreter does not support visualizing data that lives in the binary, e.g. &'static str (see also #49). For example, we should be able to visualize this program in the Rust Book:

fn main() {
let s: &'static str = "I have a static lifetime.";
}

This change will require modifying VisEvaluator::build_step to extract data held in the "binary" (wherever that is in Miri, not sure) similar to how the heap is extracted. Then the StepView will need to be updated to contain a new memory region that represents the binary.