Fixes #2538

Uses an immutable variable to store the beacon's address in BeaconProxy, in addition to the beacon storage slot. This avoids an extra SLOAD when retrieving the beacon address before each delegate call.

The immutable variable is used in _implementation() to read the beacon address before each delegate call.
The beacon slot is still written during construction, so that the beacon address can be read externally according to ERC-1967. This beacon slot is required because the immutable variable must be private to avoid generating a getter function that could clash with a function from the implementation.

Rationale:

• Although there is an upgradeBeaconToAndCall internal function in ERC1967Utils which sets the beacon slot, that function is not exposed in BeaconProxy but is only called on construction. The BeaconProxy itself does not provide a way to modify the beacon address after the BeaconProxy is deployed.
• Therefore, we should not need to continue reading the beacon slot before every delegate call during the proxy's fallback function. We can use an immutable variable instead.

PR Checklist

• Tests (not applicable as this only changes the proxy's internal implementation)
• Documentation (updated comments)
• Changeset entry (run npx changeset add)