Use immutable

[chriseth]

The proxy code has a comment that tells that immutable cannot be used in inline assembly. While this is true, I wonder why you did not "copy" the immutable value to a local variable first which is then accessed through inline assembly.

[spalladino]

My original intention was to write the proxy entirely in Yul, following this example. I never got around to that, so it makes sense to simplify the current implementation by just doing as you suggest!