Boost AM::Serializer perf

Benchmarks (in the benchmark folder) show these changes give 3x perf for serialization under some cases. 

I the real world for a big discourse page we see a 10% boost (though it was a bit bigger on 2.0 for some reason): 

BEFORE: 

```
sam@ubuntu:~/Source/discourse$ ab -n 30 -c 1 http://l.discourse:3000/t/install-tutorial-for-the-ruby-dumb-test/2027
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking l.discourse (be patient).....done


Server Software:        thin
Server Hostname:        l.discourse
Server Port:            3000

Document Path:          /t/install-tutorial-for-the-ruby-dumb-test/2027
Document Length:        57264 bytes

Concurrency Level:      1
Time taken for tests:   5.182 seconds
Complete requests:      30
Failed requests:        6
   (Connect: 0, Receive: 0, Length: 6, Exceptions: 0)
Write errors:           0
Total transferred:      1744598 bytes
HTML transferred:       1717909 bytes
Requests per second:    5.79 [#/sec] (mean)
Time per request:       172.719 [ms] (mean)
Time per request:       172.719 [ms] (mean, across all concurrent requests)
Transfer rate:          328.80 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.0      0       0
Processing:   143  173  22.4    179     215
Waiting:      143  173  22.4    179     215
Total:        143  173  22.4    179     215

Percentage of the requests served within a certain time (ms)
  50%    179
  66%    184
  75%    186
  80%    188
  90%    210
  95%    214
  98%    215
  99%    215
 100%    215 (longest request)


```


AFTER:

```
sam@ubuntu:~/Source/discourse$ ab -n 30 -c 1 http://l.discourse:3000/t/install-tutorial-for-the-ruby-dumb-test/2027
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking l.discourse (be patient).....done


Server Software:        thin
Server Hostname:        l.discourse
Server Port:            3000

Document Path:          /t/install-tutorial-for-the-ruby-dumb-test/2027
Document Length:        57097 bytes

Concurrency Level:      1
Time taken for tests:   4.911 seconds
Complete requests:      30
Failed requests:        12
   (Connect: 0, Receive: 0, Length: 12, Exceptions: 0)
Write errors:           0
Total transferred:      1739581 bytes
HTML transferred:       1712922 bytes
Requests per second:    6.11 [#/sec] (mean)
Time per request:       163.696 [ms] (mean)
Time per request:       163.696 [ms] (mean, across all concurrent requests)
Transfer rate:          345.93 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.0      0       0
Processing:   140  164  25.9    145     231
Waiting:      140  164  25.9    145     231
Total:        140  164  25.9    145     231

Percentage of the requests served within a certain time (ms)
  50%    145
  66%    182
  75%    183
  80%    184
  90%    199
  95%    213
  98%    231
  99%    231
 100%    231 (longest request)
```