NestedString representation for VSCode extension

[ranjitjhala]

Render trace using NestedString (a JSON representation) that lets the VSCode plugin display nested objects (e.g. struct with many fields) interactively

@nilehmann any clue on how to avoid the pesky

warning: the feature `fmt_internals` is internal to the compiler or standard library
  --> crates/flux-middle/src/lib.rs:14:5
   |
14 |     fmt_internals
   |     ^^^^^^^^^^^^^
   |
   = note: using it is strongly discouraged
   = note: `#[warn(internal_features)]` on by default

which is needed in nested_with_bound_vars?

[nilehmann]

@ranjitjhala To deal with the internal features thing we can make fmt_bound_vars take an &mut impl fmt::Write instead of &mut fmt::Formatter<'_>. Both Formatter and String implement fmt::Write.