The bytecode generator used to keep track of the stack depth with a
16-bit counter, which could overflow for very large BCOs, resulting in
incorrect bytecode.

This commit switches to a word-sized counter, and eagerly panics
whenever an operand is too big, instead of truncating the result.

This allows us to work around the 16-bit limitation in the case of SLIDE
instructions, since we can simply factor it into multiple SLIDEs with
smaller arguments.