Wrong documentation for `instruction_cost()`

Note: This is upstream bug, but I stumbled upon it while investigating forbidden opcodes in Casper.

Looking at the documentation of Rules::instruction_cost():

Lines 12 to 14 in 57a3b0e

    
           /// Returning `None` makes the gas instrumention end with an error. This is meant 
        
           /// as a way to have a partial rule set where any instruction that is not specifed 
        
           /// is considered as forbidden.

the part "where any instruction that is not specifed is considered as forbidden" does not seem to be fully correct.

None - which is used as indication of error - is returned only when instruction type is marked as forbidden. In case instruction type does not have metering type assigned, regular cost will be used for opcode:

casper-wasm-utils/src/rules.rs

Lines 352 to 357 in 57a3b0e

    
           impl Rules for Set { 
        
               fn instruction_cost(&self, instruction: &Instruction) -> Option<u32> { 
        
                   match self.entries.get(&InstructionType::op(instruction)) { 
        
                       None | Some(Metering::Regular) => Some(self.regular), 
        
                       Some(Metering::Fixed(val)) => Some(*val), 
        
                       Some(Metering::Forbidden) => None,

	/// Returning `None` makes the gas instrumention end with an error. This is meant
	/// as a way to have a partial rule set where any instruction that is not specifed
	/// is considered as forbidden.

	impl Rules for Set {
	fn instruction_cost(&self, instruction: &Instruction) -> Option<u32> {
	match self.entries.get(&InstructionType::op(instruction)) {
	None \| Some(Metering::Regular) => Some(self.regular),
	Some(Metering::Fixed(val)) => Some(*val),
	Some(Metering::Forbidden) => None,