feat: support legacy json abis

[Will-Smith11]

currently there is no support for legacy abis. I quickly added it, ignore the extra commits, from my fork where i also quickly patched rust keyword impl by using r#

[DaniPopes]

I think we can simplify this and flatten at serde-time in StateMutability with a custom impl. There's also constant: bool alongside payable: bool

[Will-Smith11]

does constant: bool effect the generation of the bindings at all? I totally agree with flattening it but do you think we can just ignore the constant field?

[DaniPopes]

I assume it's converted like this

match (constant, payable) {
    (false, false) => StateMutability::NonPayable,
    (false, true) => StateMutability::Payable,
    (true, _) => StateMutability::View,
}

[prestwich]

(true, true) should be an error, no? as constant is a contract not to read or write state, both being set would be an invalid ABI?

[Will-Smith11]

hey sorry was super busy with work. made the changes yall wanted

[gakonst]

@DaniPopes @prestwich what do we want to do with this PR? totally defer to you guys

[DaniPopes]

I don't think we want this

Edit: nevermind, I found an easier way to implement this: #596