The `identifier-binding` function is now senstive to the "macro
introduction" scope that's on an indentifier provided to a currently
expanding macro. Unit tests for TR need to use
`syntax-local-intorduce` to remove it, in the same way that `tc-setup`
already does.
Also, recognize a class expansion when it's wrapped with
`#%expression`, since `class` now expands that way sometimes.