Speed up xor_byte_strings by 70%

Benchmark:

```ruby
require 'benchmark'
require 'benchmark/ips'
require 'securerandom'

def xor_byte_strings(s1, s2) # :doc:
  s2_bytes = s2.bytes
  s1.each_byte.with_index { |c1, i| s2_bytes[i] ^= c1 }
  s2_bytes.pack("C*")
end

def xor_byte_strings_new(s1, s2) # :doc:
  s2 = s2.dup
  size = s1.bytesize
  i = 0
  while i < size
    s2.setbyte(i, s1.getbyte(i) ^ s2.getbyte(i))
    i += 1
  end
  s2
end

s1 = SecureRandom.random_bytes(32)
s2 = SecureRandom.random_bytes(32)

Benchmark.ips do |x|
  x.report("current"){xor_byte_strings(s1, s2)}
  x.report("new"){xor_byte_strings_new(s1, s2)}
  x.compare!
end

100000.times do |i|
  s3 = SecureRandom.random_bytes(32)
  s4 = SecureRandom.random_bytes(32)
  raise unless xor_byte_strings(s3, s4) == xor_byte_strings_new(s3, s4)
end
```

Results on ruby 2.5.1:

```
Warming up --------------------------------------
             current     6.519k i/100ms
                 new    10.508k i/100ms
Calculating -------------------------------------
             current     84.723k (_ 0.4%) i/s -    423.735k in   5.001456s
                 new    145.871k (_ 0.3%) i/s -    735.560k in   5.042606s

Comparison:
                 new:   145870.6 i/s
             current:    84723.4 i/s - 1.72x  slower
```

0 files changed, 0 insertions, 0 deletions