Limit OP_NUM2BIN to 68 bytes

[EyeOfPython]

OP_NUM2BIN can be used to generate a lot of zeros on the stack (520 bytes with 1 opcode and 3 bytes). Combined with OP_HASH256 or other opcodes, this can be quite slow.

Also, numbers will never be longer than 68 bytes, so it's kinda ridiculous to allow 520 bytes.

Why 68 bytes?

There's a neat trick for reducing the size of the preimage for the OP_CHECKDATASIG/OP_CHECKSIG transaction introspection. You use ANYONECANPAY, and then with 2 68 OP_NUM2BIN you generate parts 1. (nVersion, here 02000000), 2. (hashPrevouts, here a uint256 of 0x0000......0000) and 3. (hashSequence, here a uint256 of 0x0000......0000).

This saves around 64 bytes for a transaction and only costs 1 opcode. Reducing the OP_NUM2BIN size to 68 keeps that optimization.