Not recurse forever if missing mixin declarations

Bail out after recursing 32 times into Deferred.parse and raise syntax
error for missing declaration to provide a meaningful error message.
Otherwise the code loops until the interpreter stack end (max
recursion depth).
Don't evaluate expression in 'font: 0/0 a;'

This is the ugliest (but valid) CSS statement since sliced bread and
Bootstrap3 makes use of it in the escaped form: 'font: ~"0/0" a;'. Add
test-cases for both variants and special treatment in Expression.parse.

Oh my...