static_cg: JIT `mat` to reduce compile time

Gordian Edenhofer requested to merge jit_static_cg into NIFTy_8

This shaves off about 20% of the compile time of the static_cg in the current nifty_re demo.

This is safe because the while_loop statement within static_cg would have compiled everything anyways.

Edited by Gordian Edenhofer

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