Basic framework for profiling screen execution.
sl2: Screen profiling debugging.

This entails writing information about the decisions we make when
creating the displayables comprising the screen.
sl2: Fix debug.
sl2: Encapsulate code analysis into a single object.

This is in preparation for analyzing const/non-const inside a screen.
sl2: Frameworks for fixed-point analysis.

This doesn't do anything yet, but provides the structure for constant
analysis.