Cache calculated bump seeds in context

[armaniferrante]

Closes #1364.

Pre-requisite for #1361. This is needed for non anchor accounts that won't have the account header and so can't store bump seeds directly in the account data (and so the bump needs to be stored in some other associated account).

One can use ctx.bumps.get("<my_account_name>") to get a bump seed already searched for in the accounts context.

[billythedummy]

Given that we know which accounts require a PDA check at compile-time, would it be more efficient to generate a struct like BumpsAccountStruct instead of using a BTreeMap? For example:

#[derive(Accounts)]
#[derive(bumps)] // hypothetical proc macro for this suggestion
pub struct Example<'info> {
    #[account(
        init,
        seeds = [...],
        bump,
    )]
    pub pda_1: AccountInfo<'info>,

    pub not_pda: AccountInfo<'info>,
}

would generate

pub struct BumpsExample {
  pub pda_1: Option<u8>
} 

With a BTreeMap with string keys, there are no compile-time checks for account name refactors as well. If i change the struct field name from pda_1 to pda_2 but forget to change the code from ctx.bumps.get("pda_1") to ctx.bumps.get("pda_2"), it becomes a runtime error instead

[armaniferrante]

@billythedummy that sounds good. How would you expect to access this from the context?

[billythedummy]

so i think ctx.bumps would be the generated struct instead of a BTreeMap (so BumpsExample in this case). So instead of ctx.bumps.get("pda_1") it will be ctx.bumps.pda_1. This will probably require adding a new generic to Context for this "bumps" type. Haven't looked closely at the lang internals yet so i'm not sure how difficult it would be to implement this.