Message "Incubators"

[BenLeadbetter]

A nice addition to the crate would be some "Incubator" types (name notwithstanding), which could be fed with packets / bytes and then polled for valid messages.

Considerations

• The solution should be #![no_std] friendly
• We should accommodate parallel buffering where appropriate.
• Especially for Sysex8 messages where the spec explicitly indicates they can be streamed in parallel.
• Clock messages will probably need special handling because they can always come between packets of other message types.
• FlexData and UmpStream have explicit limits on their message size (see the specs for deets).