Feat: constants declared in impl blocks in seeds

[stegaBOB]

Problem:

As of now, constants declared in impl blocks, such as the impl block for an account struct, are not parsed in the seeds constraint field.

WARNING: unexpected seed category for var: SeedPath("CollectionPDA :: PREFIX", [])

A common pattern I've used is to declare the space and the seeds constant for a specific PDA account as a constant impl of the account struct. This PR fixes adds the necessary parsing to make this work.

IDL behavior with this PR:

...
{
  "name": "collectionPda",
  "isMut": true,
  "isSigner": false,
  "pda": {
    "seeds": [
      {
        "kind": "const",
        "type": "string",
        "value": "collection"
      },
      {
        "kind": "account",
        "type": "publicKey",
        "account": "CandyMachine",
        "path": "candy_machine"
      }
    ]
  }
},
...

The above JSON was generated with the following constraints:

seeds = [CollectionPDA::PREFIX.as_ref(), candy_machine.to_account_info().key.as_ref()],

where CollectionPDA prefix is defined as below:

impl CollectionPDA {
    pub const PREFIX: &'static str = "collection";
}

The changes in this PR only impact IDL generation with seeds, and don't touch any other logic.

[vercel]

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