Abstraction for a source of buffers to write trace data to.
At the moment this is used for the ability to both write to a file and to an in-memory buffer for tests.
However I also tried to anticipate some of the structure of how the API would work which would be required to write to a shared-memory transport involving a ring or double-buffering system.
val next_buf : ensure_capacity:int -> (Core.read_write, Iobuf.seek) Iobuf.tCalled before writing up to ensure_capacity bytes of data to the destination. Any older buffers will no longer be used after calling this function, so it's legal for a Destination to re-use Iobuf.ts to avoid allocating.
All writers are expected to update the Iobuf's lo, either manually or using Iobuf.Fill. Data will only be consumed up to the new lo.