Generate more understandable error messages for axiom use earlier in the pipeline

[lukaszcz]

Currently, the presence of axioms is checked at the end of the Core pipeline when a lot of processing has already been done. When you e.g. forget to remove todos, this can result in cryptic error messages not pointing to actual source positions, like

/core-check:1:1: error:
The function lambda_22@Kudos.MetaMask uses an axiom and thus it cannot be compiled

The presence of axioms should be checked earlier, preferably around type-checking (only when actually compiling). The errors should point to where in the source the axiom is used. This is a bit tricky, because we'd need to filter for reachable definitions at this point (but I recall there is already some infrastructure for it).

[lukaszcz]

In general, all the final target-specific checks in Core should be moved earlier in the pipeline.