Update ToWkt::write_wkt to not buffer string

[kylebarron]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.
• I ran cargo fmt

---

This avoids buffering when writing WKT to std::io::Write. It's considerably faster:

geo: write small wkt    time:   [570.44 ns 573.74 ns 578.31 ns]
                        change: [-30.487% -29.975% -29.481%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  1 (1.00%) high mild
  8 (8.00%) high severe

geo: write big wkt      time:   [2.0220 ms 2.0300 ms 2.0405 ms]
                        change: [-19.849% -19.214% -18.603%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) high mild
  4 (4.00%) high severe

[kylebarron]

I updated the PR description. This is actually considerably faster than collecting a string and then writing that to the output buffer.

[kylebarron]

I'd say I'm content with keeping the direct APIs as using std::fmt::Write because we know we're writing UTF8 content. Otherwise you'd have to write to a Vec<u8> and then parse that back to a String (or maybe trust ourselves and use from_utf8_unchecked?)

[kylebarron]

This loss of error content is unfortunate but I'm not sure of a good way around this.

[michaelkirk]

See #129