More predictable handling of excessive shifts

[bitjson]

Predictable Handling of Excessive Shift Operations

This proposal requires only that shift operations specify non-negative bit counts. All shifts greater than or equal to 0 are valid, with clear, consistent behavior: binary shifts continue to "clear" (zero-fill) bits up to and beyond the item's length, excessive right arithmetic shifts correctly result in 0 or -1 (see Behavior of Arithmetic Right Shifts on Negative Values), and left shifts fail only for results which would be rejected due to stack item length limits (matching OP_MUL; implementations may optionally include a fast-failure path).

This proposal's behavior aligns with the existing numeric operations, maximizes the predictability of shift operations (by eliminating potentially-unexpected failures due to excessive shifts by otherwise-valid VM numbers), minimizes room for implementation-specific errors (e.g. the OP_RSHIFT crash and CVE-2010-5137 OP_LSHIFT crash), and reduces overall protocol complexity.