Decide on names of ZeroTrie as_borrowed and as_borrowed_slice

[sffc]

See #4381

We have functions that return both &ZeroTrie<[u8]> and ZeroTrie<&[u8]>. How should we name them to be clear? The current names are not intended to be final.

[robertbastian]

Do these even need to be public now that we have the separate cursor type?

[sffc]

I would like to resolve the name bikeshed under the assumption that these functions are useful, which I believe they are, even if we aren't using them in the lookup PR any more.

[sffc]

Discuss with:

• @sffc
• @younies

[sffc]

impl ZeroTrieSimpleAscii<Store: AsRef<[u8]> + ?Sized> {
    pub const fn as_ref(&self) -> &ZeroTrieSimpleAscii<[u8]>
}

impl &ZeroTrieSimpleAscii<[u8]> {
    pub fn borrowed_store(self) -> ZeroTrieSimpleAscii<&[u8]>;
}

impl<Store: AsRef<[u8]> + ?Sized> AsRef<ZeroTrieSimpleAscii<[u8]>> for ZeroTrieSimpleAscii<Store> {
    fn as_ref(&self) => &ZeroTrieSimpleAscii<[u8]> {}
}

LGTM: @Manishearth @sffc @robertbastian