Regexp compile

Currently regexp literals are parsed as:

```ruby
Object.send(:new, :Regexp)
```

But it is not possible to cache the instances. So how about to use `compile` method?

https://github.com/mattn/mruby-pcre-regexp/blob/master/mrblib/pcre_regexp.rb#L2-L10

If literals are parsed as following, Regexp make memolize instances.

```ruby
Object.send(:compile, :Regexp)
```

I'm not master of ruby. So If i'm missing something wrong, please close this.