feat: remove inefficient if check on match lex

[ChrisCho-H]

I'm not sure whether it's intentional, but this pattern(match and then if) could make lexer little slower for OP_CSV and OP_CLTV(and Error::InvalidOpcode) as they aren't caught in match arms but if block like below(which causes additional check).

...
script::Instruction::Op(op) => {
                if op == opcodes::all::OP_CSV {
                    ret.push(Token::CheckSequenceVerify);
                } else if op == opcodes::all::OP_CLTV {
                    ret.push(Token::CheckLockTimeVerify);
                } else {
                    return Err(Error::InvalidOpcode(op))
                }
}

[apoelstra]

ACK aa722c2; successfully ran local tests; nice!

[sanket1729]

ACK aa722c2