Lots of restructuring prompted by
a) signature changes driven by profiling
b) implementation of FeedSourceBase.AwaitCompletion

See also #116:

• cleanup naming and signatures for Sinks
  • rename ProjectorPipeline<'T> -> Sink
  • name Ingester batch tuples
• Restructure Scheduler modules
• rename Feed.Internal to Feed.Core
• convert all Tuples to struct tuples
• change StreamSpan from a record to a type alias
• Linting e.g. remove ., ref etc
• Extensive profiling-driven impl changes