Revert "Merge pull request #185 from ISSIntel/liquid-utf8"

@ozeias @dylanahsmith @Soleone 

After reading this week about how on some platforms, regexes with `\d` are much slower than `[0-9]`, I decided to benchmark the effect of UTF8 support in liquid.

With UTF8 support:
```
Rehearsal ------------------------------------------------
parse:         6.220000   0.070000   6.290000 (  6.294562)
parse & run:  12.490000   0.150000  12.640000 ( 12.640197)
-------------------------------------- total: 18.930000sec

                   user     system      total        real
parse:         5.920000   0.040000   5.960000 (  5.965448)
parse & run:  12.960000   0.140000  13.100000 ( 13.105493)
```

Without UTF8 Support:
```
Rehearsal ------------------------------------------------
parse:         5.040000   0.060000   5.100000 (  5.102662)
parse & run:  10.940000   0.090000  11.030000 ( 11.021313)
-------------------------------------- total: 16.130000sec

                   user     system      total        real
parse:         5.080000   0.020000   5.100000 (  5.104463)
parse & run:  11.850000   0.120000  11.970000 ( 11.994411)
```

That is a significant performance regression so I've no choice but to revert the UTF8 support.

This reverts commit c5dfcd29b0937e0d84780f11f61919faa4741806, reversing
changes made to f7d1e1d0c1336a9c51d652f07e0187b429e154e4.