Pancake constants pre-processing

Currently in order to have C-like `#define var ...` to specify useful constants in Pancake code, the user has to run a C pre-processor to process these constants.
It'd be nice if Pancake can have some syntax sugar to do this. And it'd also be useful if these defined constants can be used in annotations as well.

The issue names no files, tests, or entry points to begin with. First investigate Pancake's existing syntax and annotation handling, then determine how C-like constants should be declared and expanded without the C pre-processor. Done would require an agreed syntax and behavior for constants in both Pancake code and annotations, plus corresponding tests.

Written by the indexing model from the issue text.