Allow inline tag on custom map key types that implement encoding.Text*arshaler

[edkloczko]

In both the standard and experimental json libraries, I can use a custom type that implements the encoding.TextUnmarshaler and encoding.TextMarshaler interfaces as map keys. When using the inline flag, this is no longer allowed.

I think I understand why the original proposal stated:

The inlined field must be a Go struct that does not implement Marshaler or Unmarshaler

But I'm having a hard time understanding why the string-ish behavior for map keys couldn't be applied to inline tagged maps as well.

Thank you!

[dsnet]

I believe we could expand the restrictions, but permitting keys to support encoding.TextMarshaler complicates duplicate key detection. It's a solvable problem, but probably not something to solve for the first stable release of this module. The benefit of restricted features is that we can generally expand it in the future, but not the other way around.