Add alloc and no_std features

[mycognosist]

• Adds feature flags
  • std (enabled by default)
  • alloc
  • dev

I have followed the no_std support pattern used by wasm-bindgen, ie. define a std feature which is enabled by default and conditionally provides extra functionality.

The IntoVec types are not compiled if neither the std nor alloc features are selected.

The dev flag compiles the scramblers for use with fuzzers.

[AljoschaMeyer]

Nice =)

Could you make sure that cargo doc provides docs as if all flags were enabled, and then merge?

[mycognosist]

Good thinking! That should now be the case; I have explicitly added #[cfg(doc)] for the scrambler modules and type exports.