Deprecate DecodeError

[kevinheavey]

Problem

DecodeError is just a useless wrapper around num_traits::FromPrimitive since the type_of() method isn't used

Summary of Changes

• Deprecate it and don't implement it for anything in the repo
• Remove it as a constraint from PrintProgramError

[buffalojoec]

I'm in favor, but a few important bits about deprecation here.

• In order to break less stuff downstream (and be semver compatible-ish), we can't remove the trait constraint from PrintProgramError until the trait is removed and the breaking change is introduced.
• We also shouldn't drop implementations of DecodeError for types like Pubkey until the trait is dropped as well.

With those two points respected, this trait effectively becomes a no-op for PrintProgramError, but we (and other projects besides SPL) can go and evict the deprecated trait from error implementations before the trait is dropped completely.

[kevinheavey]

Yeah thinking about it more it doesn't make sense to give people a deprecation warning when they still need to use DecodeError to implement PrintProgramError which is not being deprecated. Just need to wait for 3.0 to get rid of DecodeError I think