Add support for span suppression and coalescing, simplify span creation API via builder pattern

[mpilquist]

An idea I've been thinking about for a while. Could be useful to suppress span creation from a third party library (e.g. skunk).

Trace[F].span("getOrders", Span.Options.Suppress)(doSkunkQuery)

The above would result in all Skunk spans getting ignored entirely and all fields / events / errors ignored. Span.Options.Coalesce would instead merge all the fields / events / errors to the getOrders span.

Suppression / filtering can be done by various backends but not all. The approach in this PR allows application developers to opt-out of tracing done by a library, without requiring backend specific filtering.

This PR also switches to using the builder pattern for span creation, as otherwise we'll keep having to add various permutations of parameters to new span methods, each which breaks binary compatibility.