[BREAKING CHANGE] Support multiple major versions of foreign crates via feature flags

[fujiapple852]

Currently we have feature flag such as smallvec to enable the crate of the same name, however smallvec 1.x and 2.x have large API changes that require different impls of [To|Into]BoundedStatic.

This crate should endeavour to support both, and as such that feature flags should be suffixed with the major version.

Inspired by https://github.com/sfackler/rust-postgres/blob/d2634a46b5bc1de5176f0766626e7a9f6757f362/postgres-types/Cargo.toml#L53-L54