convert `ena` to no-std

[indietyp]

Addresses #47 and converts ena to be no-std compatible.

I have taken the liberty also to make log optional (as some crates may prefer to use tracing or another solution) and removing the bitvec.rs file (addressing #18).

I have run the crate through cargo-hack to verify that all feature permutations work as expected.

The `cargo-hack` command used

cargo hack --optional-deps --feature-powerset nextest run