Support large SLIDE instructions.
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.
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