Cairo: Remove non-standard increase_allowance and decrease_allowance from ERC20

[ericglau]

Remove safe allowance option and functions as per OpenZeppelin/cairo-contracts#881

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[andrew-fleming]

Looking good, Eric! I think there's a few other remnants of SafeAllowance to get rid of:

1, 2
3

[ericglau]

@andrew-fleming Thanks for noticing! Updated.

[andrew-fleming]

Ah another thing: it's now deprecated to add the external attribute on generated trait impls:

    #[generate_trait]
    #[external(v0)]
    impl ExternalImpl of ExternalTrait {
        foo() {}
    }

Instead of using the external attribute, we can now use abi(per_item) and add the external attr to each function within the trait e.g.

    #[generate_trait]
    #[abi(per_item)]
    impl ExternalImpl of ExternalTrait {
        #[external(v0)]
        fn foo() {}
    }

Here's an example in the lib

The alternative is to use loose functions and not define them within a trait. Having the external trait doesn't really matter AFAICT. We need to make a decision on this for the wizard @martriay @ericnordelo

[ericglau]

@andrew-fleming I made the change suggested in your comment above, and also updated to use abi(embed_v0) for interface impls.

Pending further decision on whether we should use loose functions instead, we could address that in a separate issue if needed.

[andrew-fleming]

LGTM!