[PATCH] Make the lazy/eager decision explicit instead of defaulted

This patch adds a CompilationTiming enum {Undefined, Lazy, Eager} and requires users to pass a value from it to every function that may cause the JIT to generate code.

I do NOT think we should accept this patch: It changes a lot of APIs and makes users specify the choice in many places, while I think most users really just want one choice for their whole app. There's a good argument to be made that users may want to decide certain calls should be lazily-compiled, but those are boundaries _within_ a transitive call tree, not the whole-tree choice this change allows. I'd be happy with a forced choice when creating the JIT, or with an optional choice defaulted to the safer option, but this patch seems like the wrong thing to do.