Refactor implementation of `Decode::skip` and `Decode::encoded_fixed_size`

[kalaninja]

• add Input::skip
• refactor implementation of Decode::encoded_fixed_size
• refactor implementation of Decode::skip

By default, Input::skip does an actual read and discards the result, implementors should provide a specialized implementation.

By default, Decode::skip executes Input::skip for types providing Decode::encoded_fixed_size. Otherwise, it is just calling Decode::decode., so implementors should provide a specialized implementation.

As discussed in previous attempts that I found on github, Decode::skip shouldn't validate the input data, providing a fast forward functionality instead. IMHO if a calling code wants to check for data integrity they can manually do something like let _ = Decode::decode(input)?;

Closes #208 Closes #244

[kalaninja]

@ggwpez @bkchr guys, what do you think about this?