[DRAFT] SMA 2: Hardcoded Single Signer

[Zer0dot]

Motivation

This is a followup to #119, where instead of appending a validation to bytecode, we hardcode single-signer functionality into the account, effectively cutting down on initialization cost by introducing a simple one-address initializer.

Solution

Implement a built-in fallback single-signer validation. This is a global validation, and disabling it is akin to setting the fallback signer to the zero address.

To use the fallback validation, pass a validation equivalent to bytes24(max).

Adding hooks to the fallback validation is an issue still being investigated, although it is treated mostly as a normal validation, we might only need to add a bit of branching in installValidation() if this ends up being the optimal route for SMA.

This solution saves on bytecode deployment cost, but still requires an initializer, and adds to implementation bytecode.