Improve depth-cutoff for printing HsSyn in error messages
MERGE TO STABLE The "user style" in Outputable allows us to elide large expressions when printing HsSyn, printing "..." instead. This is done by calling Outputable.pprDeeper. But there was no mechanism for trimming very long lists, which occur when using do-notation or explicit lists. This patch fixes the problem, by adding Outputable.pprDeeperList. I also made some of the pretty-printing in HsExpr rather more vigorous about increasing the depth; in particular, pprParendExpr. This should make debug prints shorter.