[enhancement] add structured traceback for fn and test failures

### What's hard to do? (limit 100 words)

When a dslx function or test fails it can be difficult to traceback why it did so: the interpreter doesn't print stacktrace or contextual value about the state that led to that particular failure.

### Current best alternative workaround (limit 100 words)

Currently the best alternative is to scatter `trace_fmt` statement along the code paths that led to the failure.

### Your view of the "best case XLS enhancement" (limit 100 words)

Ideally we would have a structured representation of the execution trace of the call graph that led to the failure w/ input and output bit vectors of each function (similar to what is proposed in #1438 for channels) and possibly values of intermediate `let` bindings.

An interesting take on this could be to annotate the IR presentation itself w/ recording of the input/output signals of each node across a given execution; which could lend itself to being nicely displayed in https://google.github.io/xls/ir_visualization/.