[stdlib] Use UInt8 as the storage type for `StringRef`

[gabrieldemarmiesse]

Note to the maintainers:

Something we can see here is that it's a pain to have other structs/methods/functions access the attributes or our containers directly. Here since I changed the .data attribute, a lot of code broke, but it should not have been the case since methods _as_ptr() and _as_uint8_ptr() were provided.

I would advise to make the attributes of our struct private and to provide public getters and setters to allow other functions to use the pointers and such in a future-proof way.

Long story short, for StringRef, that would mean:

• var data: DTypePointer[DType.uint8] -> var _data: DTypePointer[DType.uint8]
• fn _as_uint8_ptr(self) -> fn get_unsafe_ptr(self)

[helehex]

I agree with adding underscores to attributes that should not be accessed directly

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