Use size_t to avoid warning

When -Wshorten-64-to-32 is used (with LLVM) the following warning is triggered

```
hoedown/src/html_smartypants.c:220:8: warning: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Wshorten-64-to-32]
        len = squote_len(text, size);
            ~ ^~~~~~~~~~~~~~~~~~~~~~
```

This tremendous effort of a pull request fixes this.