Implement string field accessors in Rust crate

[FnControlOption]

For example, here's SymbolNode::unescaped:

pub fn unescaped(&self) -> &str {
    unsafe {
        let source = (*self.pointer).unescaped.source;
        let length = (*self.pointer).unescaped.length;
        let slice = std::slice::from_raw_parts(source, length);
        std::str::from_utf8_unchecked(slice)
    }
}

This assumes (possibly incorrectly) that string fields will always contain valid UTF-8. As an alternative, from_utf8_unchecked could be omitted by returning the plain &[u8] slice instead.

[kddnewton]

Why is the necessary?

[kddnewton]

Yeah we can't do this. Strings can be contents of regexps, xstrings, strings, and symbols, all of which can have any of the 90 source encodings that Ruby supports. Better to return a u8 slice and have the consumer handle it.