Adds sysvar_get lint

[S3v3ru5]

sysvar_lint README:

sysvar_get

What it does:

Lint warns uses of Sysvar::from_account_info and suggests to use Sysvar::get instead for
the sysvars implementing Sysvar::get function. The following sysvars implement Sysvar::get:

• Clock
• EpochRewards
• EpochSchedule
• Fees
• LastRestartSlot
• Rent

Why is this bad?

The Sysvar::from_account_info is less efficient than Sysvar::get because:

• The from_account_info requires that Sysvar account is passed to the program wasting the limited space
  available to the transactions.
• The from_account_info deserializes the Sysvar account data wasting the computation budget.

The Sysvar::from_account_info should be used if and only if the program interacts with an old program that
requires the sysvar account to be passed in CPI call. The program could avoid deserialization overhead by using
the passed Sysvar account in CPI (after verifying the ID) and using the Sysvar::get.

References:
solana_program/sysvar docs,
Anchor docs

Known problems:

None

Example:

    let clock_account = next_account_info(account_info_iter)?;
    let clock = clock::Clock::from_account_info(&clock_account)?;

Use instead:

    let clock = clock::Clock::get()?;

How the lint is implemented:

• For every item
  • If item is a struct and has #[derive(Accounts)] macro
  • For each field in the struct
    • If field is of type Ty::Sysvar(T) and T is one of Clock, EpochRewards, EpochSchedule, Fees, LastRestartSlot, Rent
      • Then report the field and suggest to T::get().
• For every function
  • If an expr in function calls T::x() where x is solana_program::Sysvar::from_account_info and
    T is one of sysvars that implements Sysvar::get() method.
    • report the call expr and suggest to use T::get().

[smoelius]

Was this for testing? Maybe this can be removed?

[smoelius]

Here also, maybe this can be removed?

[S3v3ru5]

Yes, we should remove them