Add async traits, implementations and fuzzers

[mycognosist]

• Add async traits
  • nb : Send
  • local_nb : !Send
• Add SyncToLocalNbConsumer and SyncToLocalNbProducer wrapper structs
  • These are used for the implementations listed below
• Add local_nb consumer implementations
  • Cursor
  • IntoVec
  • IntoVecFallible
  • Invariant
  • InvariantNoop
  • Scramble
• Add local_nb producer implementations
  • Cursor
  • Invariant
  • InvariantNoop
  • Scramble
• Add local_nb fuzzers
  • cursors
  • invariant
  • invariant_noop
  • scramble_consumer
  • scramble_producer

[AljoschaMeyer]

You can simply write a wrapper around a sync::Consumer that implements local_nb::Consumer. And then implement most consumers by wrapping the sync versions in that wrapper. Much fewer lines of code, and a nice utility for all users of the crate as well.

Analogous for producers as well, of course.

SyncToLocalNbConsumer and SyncToLocalNbProducer are not the greatest names, but not the worst either.

[AljoschaMeyer]

(There's little else for me to review, those wrappers would get around all that copypasted code that I'm now skipping over.)

[AljoschaMeyer]

The pipe functions don't need separate error types for sync, nb, and local, nb. Those should all use the same type, unless I'm missing something.

[AljoschaMeyer]

All looking good so far.

[mycognosist]

@AljoschaMeyer Any changes or additions you'd like to see here before merging?

I can do the consume_all() helpers in a separate PR.

[AljoschaMeyer]

Looks good.