Bump crate version to 0.16.0

[tgeoghegan]

And while we're at it, update the version table in the README, as there are no remaining known blockers for VDAF-08 support.

[tgeoghegan]

#839 is closed, so I think we have all of VDAF-08 done. Are there any other blockers to releasing prio 0.16.0 to crates.io? @cjpatton, @divergentdave: Any other API compatibility breaks we want to get in?

[cjpatton]

I have two changes to make:

1. Add flp_validity_test to test-utils #898
2. Upstream XofHmacSha256Aes128 from Daphne

I'll prepare these changes tomorrow.

[divergentdave]

This URL is unused now.

[tgeoghegan]

I have two changes to make:

1. [Add `flp_validity_test` to test-utils #898](https://github.com/divviup/libprio-rs/issues/898)

2. Upstream XofHmacSha256Aes128 from Daphne

I'll prepare these changes tomorrow.

IIUC these two changes would add items to prio, which doesn't constitute an API break, right? But in any case, I'm not blocked on releasing 0.16.0 because I can make progress up in Janus by pulling in prio via git, so I'm happy to wait for those changes to come along before we do a release. Thanks, Chris!

[cjpatton]

I have two changes to make:

1. [Add `flp_validity_test` to test-utils #898](https://github.com/divviup/libprio-rs/issues/898)

2. Upstream XofHmacSha256Aes128 from Daphne

I'll prepare these changes tomorrow.

IIUC these two changes would add items to prio, which doesn't constitute an API break, right? But in any case, I'm not blocked on releasing 0.16.0 because I can make progress up in Janus by pulling in prio via git, so I'm happy to wait for those changes to come along before we do a release. Thanks, Chris!

I think the "test-util" change in #898 ended up being API breaking.

[cjpatton]

Once we land #903 I think 0.16 is good to ship.