Adds basic constant support.

This adds compile-time support for constants (which is accomplished by defining each constant as a macro, and then informing JDI about them). Tested to work with simple constant substitution.

Note that LGM still does not save constants (in EGM). I'm hoping we can at least add the Default configuration, and maybe focus on multiple configurations later? Anyway, I'd like to at least get a discussion started on constants.