Merge #506 #507 #508

506: Fix: do not use object after it has been moved r=philberty a=dkm

After std::move(mapping), mapping is unspecified

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507: Add more info in HIR dump for struct r=philberty a=dkm

Make struct fields more visible in dumps and add the mapping in the dump string.

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508: Make 3 error messages slightly more informational r=philberty a=dkm

Add more context to some internal error messages (ie. not intended for user)

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Co-authored-by: Marc Poulhiès <dkm@kataplop.net>