It may be obvious, but I first thought initializing an BackendEnum with a certain value could be possible with `enum(FQCN, value)`, but it does not, it always returns the first case instance (if exists).
Instead, `enum(FQCN).from(value)` must be used, I think it's worth to improve `enum()` examples.
The implementation contains an optimized implementation of the function
for the common case of using a string literal as the argument. It will
validate the enum existence during compilation and compile the code to
use `MyEnum::cases()` directly.