Libublk-rs v0.1.0

Hello,

Libublk-rs(Rust)[1][2] 0.1.0 is released.

The original idea is to use Rust to write ublk target for covering all
kinds of block queue limits/parameters combination easily when talking
with Andreas and Shinichiro about blktests in LSFMM/BPF 2023.

Finally it is evolved into one generic library. Attributed to Rust's
some modern language features, libublk interfaces are pretty simple:

- one closure(tgt_init) for user to customize device by providing all
  kind of parameter

- the other closure(io handling) for user to handling IO which is
  completely io_uring CQE driven: a) IO command CQE from ublk driver,
  b) target IO CQE originated from target io handling code, c) eventfd
  CQE if IO is offloaded to other context

With low level APIs, <50 LoC can build one ublk-null, and if high level
APIs are used, 30 LoC is enough.

Performance is basically aligned with pure C ublk implementation[3].

The library has been verified on null, ramdisk, loop and zoned target.
The plan is to support async/await in 0.2 or 0.3 so that libublk can
be used to build complicated target easily and efficiently.

Thanks Andreas for reviewing and providing lots of good ideas for
improvement & cleanup. Thanks German Maglione for some suggestions, such
as eventfd support. Thanks Joe for providing excellent Rust programming
guide.

Any feedback is welcome!

[1] https://crates.io/crates/libublk 
[2] https://github.com/ming1/libublk-rs
[3] https://github.com/osandov/blktests/blob/master/src/miniublk.c

Thanks,
Ming