Migrate to 2021 edition and add examples

[urschrei]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

[lnicola]

We could go right to 2021, I think?

[urschrei]

Oh, I was just being conservative because nothing else in georust is 2021 yet

[lnicola]

Might be a good place to start, since wkt isn't tested on older compilers :smile:.

[urschrei]

Well I guess we know it doesn't work.

[lnicola]

We need something like:

    - name: Install Rust toolchain
      uses: actions-rs/toolchain@v1
      with:
        toolchain: stable
        profile: minimal
        override: true
        components: rustfmt, rust-src

[urschrei]

Should I add a doc entry for the serde feature? Is it in use? I didn't know WKT-in-JSON was something people did.

nm I did it.

[urschrei]

Two last things I'd like to nail down:

• a one-line doc comment for the Wkt struct (what's it for?)
• a one-line doc comment for the Geometry enum (what's it for?)

[nyurik]

@urschrei should this be merged? There are two approvals, but no merge, so high chance of bitrot

[urschrei]

Oh, yeah. I think I was just leaving it for a few days.

[urschrei]

bors r+

[bors[bot]]

Build succeeded:

• wkt