Finalize on feature flags vs rustc `--cfg` flags for default impl selection

[akhilles]

Context: https://github.com/mrhooray/crc-rs/issues/92#issuecomment-1972696261

Some advantages of --cfg flags:

• Don't have to be strictly additive
• Can only be set when compiling a binary (hidden to libraries)
• Can throw a compile error if incompatible flags are set
• Can stop supporting them without breaking semver