reuse amount validations

[cavemanloverboy]

Problem

Currently, both amount and lamports are validated during a transfer involving native accounts. Since amount is already validated, it is not strictly necessary to validate lamports as well.

Solution

Use unchecked operations when manipulating lamport values, which reduces CU consumption.

[cavemanloverboy]

if there is an infinite SOL mint bug on solana, this would break backwards compatibility fyi

you'd get a issue regarding non-conservation of lamports instead of overflow error

[febo]

we double check that the protocol has an explicit u64::MAX limit for the total amount of lamports in a cluster.

https://github.com/anza-xyz/agave/blob/83d40ef2be8ed0a63bbf9346d3746006da4778c5/runtime/src/bank.rs#L764

[joncinque]

Looks good to me!