Guo Xiang Tan fa35137d3a PERF: Reduce allocations in Emoji.unicode_replacements.
```
MemoryProfiler.report { Emoji.unicode_replacements }.pretty_print
```

Before:

```
Total allocated: 2032131 bytes (30219 objects)
Total retained:  798059 bytes (10734 objects)

allocated memory by gem
-----------------------------------
   1270762  discourse/app
    690149  2.4.1/lib
     48764  activesupport-4.2.8
     11405  bootsnap-1.0.0
      5266  logster-1.2.7
      2473  hiredis-0.6.1
      2056  redis-3.3.3
       840  discourse/lib
       240  activerecord-4.2.8
       176  thread_safe-0.3.6
```

After:

```
Total allocated: 1715713 bytes (22449 objects)
Total retained:  797985 bytes (10737 objects)

allocated memory by gem
-----------------------------------
    957250  discourse/app
    690028  2.4.1/lib
     48764  activesupport-4.2.8
     11445  bootsnap-1.0.0
      2745  hiredis-0.6.1
      2624  redis-3.3.3
      1473  logster-1.2.7
       928  discourse/lib
       280  activerecord-4.2.8
       176  thread_safe-0.3.6
```
2017-06-20 13:50:37 +09:00
..
2017-06-16 15:20:20 +04:30