Gracefully handle incorrect code

[Nadrieril]

When charon is given incorrect rust code, it often crashes before letting rustc display the error to the user. We should make sure we always check the code before translating it.

This is straightforward to do (simply hook into after_analysis), but we would run into stealing issues. Hence this likely requires a fully general fix to stealing issues, such as using optimized MIR or evaluated values for constants. See also hacspec/hax#27.