lang: Replace hardcoded discriminators with Discriminator::DISCRIMINATOR

[acheroncrypto]

Problem

There are some places where we're using hardcoded discriminators instead of the associated Discriminator::DISCRIMINATOR constant in codegen.

For example the following section:

https://github.com/coral-xyz/anchor/blob/c869e11a83173355123c4109d3f37add1b3bd69d/lang/attribute/program/src/declare_program/mods/cpi.rs#L76

expands to something like:

data.extend_from_slice(&[142u8, 128u8, 47u8, 77u8, 15u8, 152u8, 240u8, 42u8]);

when we could just use the associated constant to make it simpler and less error-prone:

data.extend_from_slice(MyInstruction::DISCRIMINATOR);

Summary of changes

Replace hardcoded discriminators in declare_program! with Discriminator::DISCRIMINATOR.

[vercel]

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