Don't use registers to store window configuration

Fixes #10

This simplifies the scheme greatly, by storing the window configuration in the target buffer, not a register.

For now, the old calling style is detected and generates an error at compilation time so that client code can be updated.

I personally find this code simpler and the behaviour less quirky.

Additionally, the use of a single advice identifier means that `(fullframe ...)` forms can be re-evaluated multiple times without creating multiple pieces of conflicting advice.

Give it a whirl and let me know what you think. :-)