Improve handling of bound variables when executing

[istathar]

When running a Technique we now print the values of bound variables. This picks up the notion of "echoing" them that was used when a Technique run was started and extends it to procedure invocation and also loop iteration. This feature was extended to do partial detection of the type of the value being input so that this is rendered as a string literal, numeric, or list as appropriate.

Fix behaviour of foreach so that if the input list is empty no iterations are performed.

Finally, we make a small UI improvement to the prompt when we can infer that a list value is being demanded and render [ and ] around the input cursor to hint to the user that they are in fact being asked to enter a list.

Fix the indentation of substeps when running so that they appear nested as they do in the source Technique document.

Refine output of code block in descriptives. In particular, variables with known values are substituted into the content and shown syntax highlighted during the run (distinct from interpolations, which insert the raw variable Display into the String).