Put SIMD levels behind feature flags #92
Description
While the main purpose of this crate is SIMD, it would be nice if there were an option to disable it so that only the "Fallback" level is compiled (e.g. by adding a simd feature that is enabled by default but can be disabled). Ideally, this will also allow us to add a forbid(unsafe_code) attribute in case that feature is disabled.
The use case would be crates who want to allow their users to optionally disable SIMD for 1) binary size purposes and 2) safety reasons. Although the disadvantage is that as long as one crate in the dependency chain has the simd feature activated, it will enable it for all other crates as well.