make reverse(::StringView) return String (#61579)
As commented
https://github.com/JuliaLang/julia/pull/60526#discussion_r3074716617, I
think `reverse(::StringView)` should return `String` instead of
`StringView{Memory{UInt8}}`.
It can't generally return the same type of `StringView` anyway (because
the original `data` array type might not be mutable), it has to make a
copy anyway, and so my feeling is that we might as well return `String`.
This allows the implementation to be merged with the nearly identical
implementation for `String`.
Returning `String` also has the advantage that the result can be passed
to a `ccall` expecting NUL termination without making a copy. As
discussed in
https://github.com/JuliaLang/julia/issues/6165#issuecomment-124128940:
> we need a reverse function for only one reason: efficient
reversed-order string processing with external libraries, especially
PCRE.
so the main reason for `reverse` on a string is, in fact, passing to
`ccall`.
(This is not breaking because the `StringView` feature has not yet been
released. Also note that the StringViews.jl package has returned
`String` from `reverse` for some time now and no one has complained.)
