Dex has a reactive notebook and literate programming environment in `dex web`.

Tooltips add additional IDE information to the web environment, shown when
hovering the cursor over code.

This initial PR adds infrastructure for tooltips, with parse tree visualizations
as the first feature.

- Backend: compiler computes a `SpanTree` describing all nested expression
  spans, based on concrete syntax trees.

- Frontend: HTML rendering uses `SpanTree` to augment HTML elements with class
  attributes about span ranges. These span range attributes are highlighted on
  mouse hover.

Co-Authored-By: Dougal <d.maclaurin@gmail.com>

`Typeable` instances were added to implement an older version of
TraverseSourceInfo using reflection to find all nested `SrcPosCtx` values.

This is no longer needed, as TraverseSourceInfo now only uses `Generic`, which
is more lightweight.