trace: add cursor to for bsym insertion

Nobody has claimed this yet.

currently bsyms are appended to the trace. for transformations, we usually have a place in mind where these should go instead.
the current way is to

```
trace.push_scope([])
out = sym(foo, bar)
...
trace.bound_symbols[i+1:i+1] = trace.pop_scope()
```

pitch: we should get a cursor to insert bsyms at a given location directly.
```
trace.set_cursor(i)
out = sym(foo, bar)
...
```

two options:
1) use context manager - pro: backward compatible con: more indentation
2) use method set_cursor - pro: simpler for new things, con: what is with things that rely on appending to the end

I would lean to option 2) , asserting when we append to bound symbols automatically and run the tests to find the call sites where this happens.

cc @apaz-cli

Start by locating the trace implementation, trace.bound_symbols, trace.push_scope, and the call sites that append or splice bound symbols. Run the relevant test suite to identify append-dependent behavior; done means a cursor can insert bound symbols at a chosen location without breaking existing append behavior.

Written by the indexing model from the issue text.