Improving Solana account management

[LucasSte]

• [x] https://github.com/hyperledger/solang/pull/1254
• [x] It should be possible to pass AccountMetas to constructors. This entails refactoring Instr::Constructor for Solana and maybe unify it with Instr::ExternalCall. #1271
• [x] @payer must not support literals.
• [x] When we have information about the accounts, create a new AccountMeta vector (using the proper order!) and pass it to the call, using sol_invoke_signed.
• [x] The account declared in @payer must be accessible via tx.accounts
• [x] If the parameter of an annotation is a constructor argument, the annotation should appear before the argument declaration so we do not confuse this with an account declaration, which has nothing to do with arguments.
• [ ] All accounts declared must be validated (check if signer and/or mutable bits are properly set and ensure we have all the accounts needed).
• [x] Provide @signer, @reader, @mutable and @mutableSigner so that developers can specify the accounts they need. These accounts must be accessible via tx.accounts.
• [x] Account declaration can happen in external functions that have a body.