include type information in panic messages

[nikpivkin]

This PR enriches the panic message by adding type information, which will make debugging easier.

[apparentlymart]

Thanks for working on this @nikpivkin.

The two panic messages you have changed here should be completely unreachable unless there is a bug in cty. Did you find a situation where those panics would occur when you called those functions with specific input? If so, I'd prefer to fix the bug that allowed this to panic than to change the panic messages.