Printing large values is not stack-safe

[jpolitz]

When printing large, nested objects, the printing algorithm holds on to state in a while worklist and makes recursive calls that can reach user callbacks, which can trigger stack operations. Pretty-printing a large AST of thousands of nodes in constructor notation can have unexpected results because of this.

The toReprJS function needs to be rewritten to take this into account (I believe @thejakeellis deserves credit for predicting this in early Jan). It currently uses safeCall to guard the callbacks, but the context of the safeCall is not stack-safe (it has a non-empty continuation that isn't made explicit). As a result information can be lost or completely erroneous results (e.g. a JavaScript string instead of a Pyret string) can be returned.

[blerner]

Most likely closed by 7098cc2. Some more testing is needed to be sure, but I have definitely gotten correct output in stack-overflowing situations using this algorithm that did not succeed with the previous one. The compiler still builds and passes all tests, so no regressions there...