Allow pattern matching on Schema for tuples, eithers

[jdegoes]

Tuples and eithers (products/sums) are so core to schema that we should bake them in and allow pattern matching against them, rather than encoding them atop Record / Enum.

[jdegoes]

A hint getting started:

final case class SchemaTuple2[A, B](left: Schema[A], right: Schema[B]) extends Schema[(A, B)]
final case class SchemaEither2[A, B](left: Schema[A], right: Schema[B]) extends Schema[Either[A, B]]

This way, if you have a Schema[A], you can match against it to see if it's Tuple or Either, and then knowing this, you can extract out left / right sides as appropriate.

[sviezypan]

let me take this one