lang: Add `Event` utility type to get events from bytes

[acheroncrypto]

Problem

It's not easy to create an unknown event from bytes as described in https://github.com/coral-xyz/anchor/issues/2885.

Summary of changes

• Add utils module to the declare_program! macro.

• Add autogenerated Event type inside the utils module:

  /// An enum that includes all events of the declared program as a tuple variant.
  ///
  /// See [`Self::try_from_bytes`] to create an instance from bytes.
  pub enum Event {
      SomeEvent(SomeEvent),
      OtherEvent(OtherEvent),
  }
  
  impl Event {
      /// Try to create an event based on the given bytes.
      ///
      /// This method returns an error if the discriminator of the given bytes don't match
      /// with any of the existing events, or if the deserialization fails.
      pub fn try_from_bytes(bytes: &[u8]) -> Result<Self> {
          // ...
      }
  }

Resolves https://github.com/coral-xyz/anchor/issues/2885

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