Use item name in error/warning messages instead of hashes

[pkolaczk]

error: Script execution error: missing instance function `0x15b77c7748430d90` for `Session` (at inst 3)

Would be nice to present the original identifier instead of its hash. I guess this would require keeping the mapping somewhere just for the purpose of error reporting.

[udoprog]

Hm. It should be possible if we have debug info available in the unit (https://docs.rs/runestick/0.9.1/runestick/struct.Unit.html#method.debug_info) which requires the debug-info option to be enabled during compilation (https://docs.rs/rune/0.9.1/rune/struct.Options.html#method.debug_info).

EmitDiagnostics could be modified to take this into account somehow if available.

[jasal82]

+1. Definitely needed.