fix: remove DecodeError from trait impls

[therealdannzor]

Summary

Resolves #73 and continues with similar work, in the same spirit, as the PR in solana-sdk (anza-xyz/solana-sdk#104).

The changes have been specifically targeted to the trait implementations in order to make the consumption of the crate easier.

Motivation

Unless DecodeError has a particular use-case that I've missed to appreciate, this satisfies the much stricter trait solver introduced in Rust 1.84.0, and helps when implementing error traits.

As it stands now, the issue above is a blocker for upgrading to more recent Solana crates in aggregate, i.e. having dependencies which in turn imports other crates, where spl-pod is a common denominator in the dependency forest.

Note

DecodeError is still left (and used) as a dependency due to necessary for the (current) PrintProgramError and in the macro impls, together with the unit tests. Removing it entirely, like in the PR above, requires further work with adjusting the dependency tree.

This should not be breaking, rather the opposite, in allowing newer Rust compilers to interact with the crate (directly or in-directly), and not hit the error as referenced in the issue above.

[therealdannzor]

Tagging some maintainers and usual suspects, hopefully not too intrusive ping 🥹

cc @2501babe @joncinque @kevinheavey @febo