Improve the performance of string xor operation

Use `each_byte` instead of `bytes` to speed up string xor operation and
reduce object allocations.

Inspired by commit 02c3867882d6d23b10df262a6db5f937ca69fb53.

``` ruby
require 'benchmark/ips'
require 'allocation_tracer'

a = 32.times.map { rand(256) }.pack('C*')
b = 32.times.map { rand(256) }.pack('C*')

def xor_byte_strings1(s1, s2)
  s1.bytes.zip(s2.bytes).map { |(c1,c2)| c1 ^ c2 }.pack('c*')
end

def xor_byte_strings2(s1, s2)
  s2_bytes = s2.bytes
  s1.bytes.map.with_index { |c1, i| c1 ^ s2_bytes[i] }.pack('c*')
end

def xor_byte_strings3(s1, s2)
  s2_bytes = s2.bytes
  s1.each_byte.with_index { |c1, i| s2_bytes[i] ^= c1 }
  s2_bytes.pack('C*')
end

fail if xor_byte_strings1(a, b) != xor_byte_strings2(a, b)
fail if xor_byte_strings1(a, b) != xor_byte_strings3(a, b)

Benchmark.ips do |x|
  x.report('xor_byte_strings1') { xor_byte_strings1(a, b) }
  x.report('xor_byte_strings2') { xor_byte_strings2(a, b) }
  x.report('xor_byte_strings3') { xor_byte_strings3(a, b) }
  x.compare!
end

Tracer = ObjectSpace::AllocationTracer
Tracer.setup(%i{type})
p xor_byte_strings1: Tracer.trace { xor_byte_strings1(a, b) }
p xor_byte_strings2: Tracer.trace { xor_byte_strings2(a, b) }
p xor_byte_strings3: Tracer.trace { xor_byte_strings3(a, b) }
```

```
Warming up --------------------------------------
   xor_byte_strings1    10.668k i/100ms
   xor_byte_strings2    11.814k i/100ms
   xor_byte_strings3    13.139k i/100ms
Calculating -------------------------------------
   xor_byte_strings1    116.667k (± 3.1%) i/s -    586.740k
   xor_byte_strings2    129.932k (± 4.3%) i/s -    649.770k
   xor_byte_strings3    142.506k (± 4.2%) i/s -    722.645k

Comparison:
   xor_byte_strings3:   142506.3 i/s
   xor_byte_strings2:   129932.4 i/s - 1.10x slower
   xor_byte_strings1:   116666.8 i/s - 1.22x slower

{:xor_byte_strings1=>{[:T_ARRAY]=>[38, 0, 0, 0, 0, 0], [:T_STRING]=>[2, 0, 0, 0, 0, 0]}}
{:xor_byte_strings2=>{[:T_ARRAY]=>[3, 0, 0, 0, 0, 0], [:T_DATA]=>[1, 0, 0, 0, 0, 0], [:T_IMEMO]=>[2, 0, 0, 0, 0, 0], [:T_STRING]=>[2, 0, 0, 0, 0, 0]}}
{:xor_byte_strings3=>{[:T_ARRAY]=>[1, 0, 0, 0, 0, 0], [:T_DATA]=>[1, 0, 0, 0, 0, 0], [:T_IMEMO]=>[2, 0, 0, 0, 0, 0], [:T_STRING]=>[2, 0, 0, 0, 0, 0]}}
```

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