Dynamic.dict decoder needs more informative errors

[sporto]

When a value fails to decode when using dynamic.dict, we get an error that says values.

e.g.

Error(DecodeError(expected: "another type", found: "Object", path: ["values"]))

If you have dozens of values to decode, this doesn't give any information about which value failed.
CrowdHailer/oas#1

I would like to modify this decoder to return the name of the key.

[lpil]

Sounds good!

[sporto]

In #741 I tried to add more information by adding an index like values[2]. But that doesn't work well.

The name of the key is available, but only if the key decoder is for a string.
Otherwise the key can be anything. Don't know what could be a good strategy for this.
We could use string.inspect perhaps.

[lpil]

The module already defines logic for turning any value into a string for use in the error path, we'd use that again.