Decide whether to expose `godot_char_string` publicly

[chitoyuu]

Created during survey of commented code (#377).

godot_char_string is a Godot string type representing a byte string of some encoding. It is currently used internally in the #[doc(hidden)] Utf8String type in the conversion process from GodotString to the Rust String.

Reasons we might want this type to become public (likely with a re-designed API):

• Sake of completeness.
• Beyond UTF8, Godot also exposes functions to create char_strings in ascii and ascii_extended encodings, that could be interesting.

Reasons we might want to remove it altogether in favor of inline FFI calls in conversion:

• Extremely narrow API surface of the type that essentially consists of only a &[c_char] getter.

[Bromeon]

For reference, here's the godot-cpp API:

class CharString {
	friend class String;

	godot_char_string _char_string;

public:
	~CharString();

	int length() const;
	const char *get_data() const;
};

class String {
	...
public:
	CharString utf8() const;
	CharString ascii(bool p_extended = false) const;
}

If this is added, it would need some good documentation to differentiate it from GodotString and StringName. Also, might be rare enough to be hidden in some nested module...

[chitoyuu]

It might also be confusing that a CharString won't let you access any Rust chars. Perhaps it's best renamed to ByteString to make that clear?

I agree that it should probably stay out of the prelude at the very least.