Support native schema for Map

[jdegoes]

Currently there is no native support for maps. However, some protocols and formats have a native representation for maps (e.g. DynamoDB). Consequently, there is a strong advantage to having a native map type, distinct from Sequence.

• Add new Schema.Map case class, which stores a schema for key, stores a schema for value, and extends Schema[Map[K, V]].
• Support new schema type for all existing codecs and other operations that match on schema

@thinkharderdev Contender for ZIO Hackathon??

[pierangeloc]

I'd like to work on this one

[thinkharderdev]

I'd like to work on this one

Awesome! Let me know if you have any questions!