Add Kernel#String

I ported to mruby from ruby:
rb_String(VALUE val)
https://github.com/ruby/ruby/blob/trunk/object.c#L3010

rb_check_string_type
https://github.com/ruby/ruby/blob/trunk/string.c#L1745

I fixed doc.

ruby
```
 *  Converts <i>arg</i> to a <code>String</code> by calling its
 *  <code>to_s</code> method.
```

mruby
```
 *  Returns <i>arg</i> as an <code>String</code>.
 *
 *  First tries to call its <code>to_str</code> method, then its to_s method.
```