2617 - better error messages

When we stash a value, mu does several levels of work for us:

a) First it inserts instructions above the stash to convert the value to
text using to-text-line.
b) to-text-line calls to-text. Both are shape-shifting, so multiple
levels of specialization happen.

To give a good error message, we track the 'stack' of current
specializations at the time of the error, and also check if the
offending instruction at the top-most level looks like it was inserted
while rewriting stash instructions.

Manual example (since booleans can't be stashed at the moment):
  x:boolean <- copy 1/true
  stash x

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