Fix compiler warnings

This fixes a few compiler warnings, including:

* implicit function declaration (`import Utils` fixed this)
* Some signedness conversions (`as UChar*` fixed these)
* Size conversion (`SizeT*` fixed these)

One warning remains, which I can't fix since I don't know anything about ooc's const capabilities:
```
source/yaml/Emitter.ooc:39:33: warning: initializing 'lang_Character__CString' (aka 'char *') with an
      expression of type 'const char *' discards qualifiers [-Wincompatible-pointer-types]
        lang_Character__CString error = (*((((yaml_Emitter__EmitterStruct*) (this->emitter))))).problem;
                                ^       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

This was with rock 0.9.9, the command used was:

`rock -x -v --cc=clang samples/utils-demo.ooc`