feat: support native constants

[DavePearce]

The goal here is to allow a constant to be defined without giving it an actual value. Notes:

• (Constraints) This constant can then be compiled into generated constraint packs (i.e. bin files), with the actual value being provided as part of constraint expansion (e.g. via the CLI when running go-corset check, or directly via the expansion API from within the prover).
• (Trace Generation) On the trace generation side, native constants would be compiled as static fields in the relevant Trace.java file (as opposed to final static fields). Potentially, they could be given a default value. Eitherway, they could then be assigned by the tracer before trace generation begins..

A plausible syntax would be :

(defconst GAS_LIMIT_MAXIMUM native)

or we have a special construct, such as:

(defnativeconst GAS_LIMIT_MAXIMUM)

(or perhaps defexternconst or just defextern or defnative).

@OlivierBBB FYI.

[DavePearce]

NOTE: you'd probably need to include a type for a native constant as, otherwise, it wouldn't know what type to use.