hygiene: @cucall is unusable from other modules

[StefanKarpinski]

As I noted here, the @cucall macro has serious hygiene issues. Calling eval inside of a macro is inherently problematic since the expression comes from the caller of the macro but eval evaluates it in the defining module, not in the caller's context. A much lesser problem is that it doesn't call esc on the expressions passed in by the caller; however, it's hard to fix that until eval issue is fixed.