Allow ByteArray as input when parsing JSON

[yorickpeterse]

The JSON parser introduced in master only supports String as input, which makes parsing files rather wasteful/clunky as you first have to convert a ByteArray to a String. Instead the JSON parser should just support both String and ByteArray.

For this to work we probably need to introduce a trait that is both implemented by String and ByteArray, and use that as the expected input. The parser does try to parse strings into Int/Float values (with a custom offset) in a few places, so we need a way of handling that.

[yorickpeterse]

Any runtime library changes that need to be made will probably conflict with #508, so it's best to wait until said PR is merged.

[yorickpeterse]

There are two approaches I can think of:

1. We just take a ByteArray as input. When parsing from IO inputs, this is probably the type of data you already have anyway. String also implements IntoByteArray, so we can take that as an argument.
2. We introduce some sort of Slice type which is a class enum with two cases: String and ByteArray, wrapping a reference to their corresponding types. We then provide the necessary methods for creating sub slices, turning it into a String, etc.

In both cases a problem is that std::json::Parser uses _INKO.string_to_int and _INKO.string_to_float directly, which only supports strings as input. We'd have to change these intrinsics to operate on bytes directly, then pass those in accordingly.