lang: Fix instructions with no accounts causing compilation errors when using declare_program!

[acheroncrypto]

Problem

Instructions with no accounts result in compilation errors when they're used with declare_program!.

For example, using an IDL that has an instruction like the one mentioned in #3514 (comment)

{
  "name": "refresh_reserves_batch",
  "discriminator": [144, 110, 26, 103, 162, 204, 252, 147],
  "accounts": [],
  "args": [
    {
      "name": "skip_price_updates",
      "type": "bool"
    }
  ]
}

results in:

error[E0107]: struct takes 0 lifetime arguments but 1 lifetime argument was supplied
 --> programs/name/src/lib.rs:5:1
  |
5 | declare_program!(kamino);
  | ^^^^^^^^^^^^^^^^^^^^^^^^- help: remove these generics
  | |
  | expected 0 lifetime arguments
  |

A similar issue (#1628) with the cpi feature has existed roughly since the creation of Anchor, but this one is specifically for the cpi module generated using declare_program!. The benefit of using the IDL + macro approach here is that we have access to all instruction information (unlike with the cpi feature, where we only have access to the token stream), which we can then use to decide whether to add the generic lifetime or not.

Summary of changes

Fix instructions with no accounts causing compilation errors when using declare_program! by conditionally adding the lifetime parameter only when the instruction accounts are not empty.

Fixes #3514

[vercel]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.