Implement rest of RFC 1022: Typing free objects & simplifying SDL syntax

[msullivan]

We only did about half of it.
https://github.com/geldata/rfcs/blob/master/text/1022-freetypes.rst

The core of the feature will probably not be too bad to do.

The hard part is that I bet we will start hitting a bunch of materialization bugs when we try using it, and might need to do a heavy rework there to make it work well enough.

Probably someone else will do the core feature work while I work on overhauling materialization?