feat(infra): introduce automated publishing

[miraclx]

This PR introduces a CI job that automates versioning and publishing borsh-rs crates to crates.io.

Here's what the tag page should look like:

Here's what the release page should look like:

[matklad]

Kicked auto-merge -- let's fix nits after we make sure the thing works as intended.

[austinabell]

Unversioning everything to have all of this happen in a separate PR? I'm a little confused about why this is necessary

[miraclx]

@austinabell, unversioning everything in the source code because now all crates share the version in the root Cargo.toml. The version only needs to be updated in one place in a PR and CI fills it in at the time of publish for all the member crates.

[matklad]

I find github releases useful if you want to subscribe to releases.

Hm, yeah, looking around it seems that, eg, serde is doing releases https://github.com/serde-rs/serde/releases, so this is a strong evidence that I am wrong, and that folks do actually find them useful. Than, yeah, we should add "create gh release" to the automation.

[frol]

I have resolved the Travis issue (it was explicitly set in the branch protection rule on github to wait for Travis checks)

@miraclx Please, merge once you feel ready, the PR is unblocked

[miraclx]

Great. I still have some unpushed commits (creating release works, but I'm touching on it a bit so it's WIP). I'll update on the status in a sec.

[miraclx]

This looks good to me now. @frol, @matklad, @austinabell please take a second look.

[miraclx]

Glad to announce: the release passed successfully.

Release Page: https://github.com/near/borsh-rs/releases/tag/v0.9.2

• Upgrade hashbrown from 0.9 to 0.11. This can breakage in the rare case
  that you use borsh schema together with no-std support and rely on a specific
  version hashbrown of SchemaContainer. This is considered to be obscure
  enough to not warrant a semver bump.

Crate Links

• borsh: https://crates.io/crates/borsh/0.9.2
• borsh-derive: https://crates.io/crates/borsh-derive/0.9.2
• borsh-derive-internal: https://crates.io/crates/borsh-derive-internal/0.9.2
• borsh-schema-derive-internal: https://crates.io/crates/borsh-schema-derive-internal/0.9.2

Now we just need to make sure the CHANGELOG stays updated before each new release.