Make tests build

[ggwpez]

Tests do not build with only MEL feature as pointed out in #428 (review)

This standard pattern that is used multiple times in the code does not actually work:

#[cfg(not(feature = "derive"))]
use parity_scale_codec_derive::Decode;
use parity_scale_codec::Decode;

It errors with a redefinition of Decode when the feature is disabled. I aliased them in the tests now to DeriveDecode.
It is a bit unfortunate that parity_scale_codec::Decode is both a trait and a derive macro, depending on the features.

[bkchr]

I'm not happy with this. People will be confused on where DeriveEncode is coming from.

https://users.rust-lang.org/t/how-to-document-optional-features-in-api-docs/64577/3 above the code example with feature = "derive" would be really nice!

[ggwpez]

This auto-generated feature warning seems to only work on whole modules or types. Not on single doc lines, so i added:

#![cfg_attr(feature = "derive", doc = "```rust")]
#![cfg_attr(not(feature = "derive"), doc = "*(Only compiled with feature `derive`)*\n```ignore")]

To each example to not compile them without the feature. Works also on stable.

[mrcnski]

It errors with a redefinition of Decode when the feature is disabled.

Instead of the macro import could you have renamed the trait import, or even brought it in without a name, e.g. use Decode as _? But this is fine too.

[ggwpez]

Instead of the macro import could you have renamed the trait import, or even brought it in without a name, e.g. use Decode as _?

I tried both, but the trait and the derive macro have the same name. So when i import the trait as _, then the derive macro also becomes unavailable 🤦‍♂️
And when i import the derive macro as _ from the derive crate, then it is not usable.