v1.16.0 release preparation

[alejandro-vaz]

1. produce the changelog for 1.15.2 -> 1.16.0 keeping how changelogs were already made in https://github.com/servo/rust-smallvec/releases
2. verify documentation builds
3. create tag + release

this release is a bit premature and it's mainly because there's no documentation for 1.15.2: https://docs.rs/crate/smallvec/latest

the few commits on it already fix that, but they also remove a feature (debugger visualizer) so this has to be a new minor version

this was initially proposed by @atouchet in #415

note that the release is scheduled for the 31st