WIP: Free up curly brace syntax.

The fate of the `{}` syntax has been debated in #7941, #7128, #2488, #6739 and many other threads that I cannot dig up at the moment.  My suggestion was outlined by Jeff in #8470.  Whatever we decide to do, this change makes it easier to transition to allowing `[a, b]` to be non-concatenating (#3737)  and regularization of `Dict` syntax #8521.

The parser should probably be changed to reject the `{}` array / dict syntax during the transition period.  Would throwing an error be sufficient?  This will be a hugely breaking change so I thought I would solicit some feedback before finishing that part of the PR.