Optimize implementation of `Decode::skip()` and `Decode::encoded_fixed_size()` for most primitives types

related https://github.com/paritytech/parity-scale-codec/issues/201
related https://github.com/paritytech/parity-scale-codec/issues/241

Do a proper implementation of `Decode::skip()` and `Decode::encoded_fixed_size()` for most primitives types.