lang: Export Discriminator trait from prelude

[acheroncrypto]

Problem

Discriminator trait is required to be in scope to get access to the data type's discriminator, but it requires manually importing the trait because it's not exported from prelude.

Summary of changes

• Export Discriminator trait from prelude
• Remove redundant Discriminator trait import in codegen
• Remove full paths for the Discriminator trait from IDL's discriminator field generation

[vercel]

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