Add stacktrace functionality and replace (some) uses of Machine.String with it

[thehowl]

          From the review meeting: the agreed solution is to switch from doing `Machine.String()` to printing a stacktrace. (This is likely to be more useful and parseable information by the programmer anyway... :) )

Originally posted by @thehowl in #1736 (comment)

The Machine.String representation is sometimes too verbose or costly to produce to be useful/efficient. A better approach is to print a stacktrace, in a similar fashion to go programs, when for instance panicking within the chain, or within gno test/run