Fix sliced_string_as_bytes FP with a RangeFull

[yotamofek]

changelog: [sliced_string_as_bytes]: don't fire on str[..].as_bytes()

So I ran into this in some codebase I was working on,
where the lint fired on this line:

let string: &str;
string[..].as_bytes()

So I was trying to understand the rationale behind this lint, and it says:

It involves doing an unnecessary UTF-8 alignment check which is less efficient, and can cause a panic.

This is obviously not true in the case where a RangeFull slice is being taken, since there is no UTF-8 boundary check, and no panic can be caused. So I created an exemption for RangeFulls.

Two other notes:

1. I'm not sure the word "alignment" in the lint's description (quoted above) is really correct, should probably say "char boundary" instead?
2. I might be missing something, but isn't there a lint for doing superfluous slice indexing, and then calling a slice method? e.g. str[..].len() or slice[..].len(), where str and slice are &str and &[T], respectively. If we had one, I would expect it to fire for the example code I quoted above.

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