Freeze static strings used when writing a zipfile

As mentioned in rubyzip/rubyzip#130, `#freeze` many static Strings in `entry[_set].rb`, `extra_field.rb`, and `file.rb` for better performance in ruby >= 2.1. Performance should not be hindered in ruby < 2.1.

Performance is improved by reducing the number of String allocations that need to occur, thus reducing GC time. Here is a before and after:

### master

```
$ ruby rubyzip_write.rb | grep "String"
                       sourcefile                          sourceline       class       count
---------------------------------------------------------  ----------  ---------------  -----
/Users/srawlins/code/rubyzip/lib/zip/entry_set.rb                  80  String           10050
/Users/srawlins/code/rubyzip/lib/zip/extra_field.rb                67  String            7000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     365  String            5044
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     265  String            4000
/Users/srawlins/code/rubyzip/lib/zip/entry_set.rb                  21  String            3840
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     107  String            3000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                      62  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                      48  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     254  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     273  String            2000
/Users/srawlins/code/rubyzip/lib/zip/deflater.rb                   20  String            2000
/Users/srawlins/code/rubyzip/lib/zip/extra_field.rb                73  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     434  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                      63  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     275  String            1920
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     458  String            1000
/Users/srawlins/code/rubyzip/lib/zip/file.rb                      261  String            1000
/Users/srawlins/code/rubyzip/lib/zip/central_directory.rb          29  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     360  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     301  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     355  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     367  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     637  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     181  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                      54  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     513  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     378  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     411  String            1000
...
# totalling 63854 allocations from these locations
```

### this pull request

```
$ ruby rubyzip_write.rb | grep "String"
                       sourcefile                          sourceline       class       count
---------------------------------------------------------  ----------  ---------------  -----
/Users/srawlins/code/rubyzip/lib/zip/extra_field.rb                67  String            7000
/Users/srawlins/code/rubyzip/lib/zip/entry_set.rb                  80  String            5050
/Users/srawlins/code/rubyzip/lib/zip/entry_set.rb                  21  String            3840
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     365  String            3044
/Users/srawlins/code/rubyzip/lib/zip/extra_field.rb                73  String            2000
/Users/srawlins/code/rubyzip/lib/zip/deflater.rb                   20  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     273  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     265  String            2000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     275  String            1920
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     378  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     434  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     458  String            1000
/Users/srawlins/code/rubyzip/lib/zip/central_directory.rb          29  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     513  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     367  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     360  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     181  String            1000
/Users/srawlins/code/rubyzip/lib/zip/entry.rb                     355  String            1000
...
# totalling 37854 allocations from these locations
```

GC time is down too:

```
GC 16 invokes.
Index    Invoke Time(sec)    Use Size(byte)    Total Size(byte)    Total Object           GC Time(ms)
    1               0.220           1328360             3688320           92208    4.9319999999999915
    2               1.118          19905120            37389120          934728   86.0360000000000013
    3               1.686          19906480            37389120          934728   35.1129999999999498
```

vs

```
GC 16 invokes.
Index    Invoke Time(sec)    Use Size(byte)    Total Size(byte)    Total Object           GC Time(ms)
    1               0.221           1328360             3688320           92208    5.6819999999999923
    2               1.032          15745440            30094080          752352   77.9510000000001070
    3               1.506          15746800            30094080          752352   30.0819999999999438
```