Rename UTF16Item[s] to Utf16Item[s]

[Sawyer47]

According to Rust's style guide acronyms should be CamelCase.

[Sawyer47]

There is also OSRng and FPCategory, but I didn't want to change them without asking.

[lilyball]

Given the ubiquity of "UTF" it looks weird to me to see "Utf".

[Sawyer47]

I agree, but on the other hand "everything should be CamelCase" rule makes sense. "Uuid" or "Html" looks equally weird to me.

[kud1ing]

The style guide currently states Acronyms should be camel case, too: Uuid, not UUID.
https://github.com/mozilla/rust/wiki/Note-style-guide

[alexcrichton]

I'm in favor of this because it is following our existing conventions, just bringing some old code up to date. This will break previous users of these items, so can you flag this change as a breaking change as described here?

[Sawyer47]

Commit message changed. What about other names that don't follow this rule? Should I create an issue to collect all other type names that may need renaming?

[alexcrichton]

Either an issue or a PR is ok, incremental changes are perfectly ok as well!