refactor: switch to array-init

- Use array-init for the lowest level vector operations. Makes them nicer and less bug-prone, with zero performance impact.
- Remove unnecessary/unused ::new() functions on Vector and HeightVector.
- Update README with new dependency.
- Fix typo in README.

Signed-off-by: Stephen Waits <steve@waits.net>

Cargo.toml

@@ -36,6 +36,7 @@ name = "million-updates"
 harness = false
 
 [dependencies]
+array-init = "2.1.0"
 num-traits = "0.2.15"
 rand = "0.8.5"
 serde = {version = "1.0.152", features = ["serde_derive"]}