Allow non-string while string-able values as map keys

[acruise]

Hey folks, this library looks pretty good so far, but I'm running into one awkwardness around Map properties keyed by something other than String.

When the key type is a data class in my own codebase, I can annotate it with a serializer with a primitive descriptor, and avro4k is happy to use that. But for an external type, I'd like to be able to have properties like Map<@Contextual SomethingExternal, Foo> and have avro4k look up the serializer for SomethingExternal dynamically from context, but it doesn't seem to do so.

I can set @file:UseSerializers on the file, but I'd rather use @Contextual. Thanks!

[vladimirfx]

Provide a simple example - I try to fix this.

[thake]

@acruise thanks for raising this issue. Can you provide a small code (i.e. a failing unit test) so we can work on a solution more easily?

[Chuckame]

Just a side note. Regarding performances, you should use @useserializer instead of contextual, because contextual is resolving the serializer at runtime, while @useserializer is resolved at compile time!

[Chuckame]

See #148