Add `mapK` to `Tracer`, `Tracer.Meta`, `SpanBuilder`, `SpanOps`,
`Span`, `Span.Backend`, and `InstrumentMeta`.

Add `KindTransformer` type used by `mapK` implementation, allowing
for a single `mapK` implementation that supports many
transformation target types (`OptionT`, `EitherT`, `IorT`, `StateT`,
`Kliesli`, `Resource`) with very little work needed to add
additional or custom ones.

remove Nested instance