Can't use identifiers program_id or accounts in handler names

[armaniferrante]

The codegen uses these names before invoking the handler. Instead, we should add a __ prefix to all variables used by the codegen so that there's no collision.

[armaniferrante]

Related #347.

As pointed out by @NorbertBodziony, one can't use the word "nonce" in the zero copy state constructor.

 pub fn new(
            &mut self,
            ctx: Context<New>,
            nonce: u8,
        ) -> ProgramResult {}

[acheroncrypto]

Fixed in #2464.