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perf(builtin): Iter::join accepts StringView separator

The other members of the join family — `Array::join`, `ArrayView::join`,
`FixedArray::join`, `ReadOnlyArray::join`, `Deque::join` — already take
`separator : StringView`. `Iter::join` was the outlier, taking `String`,
which forced every StringView caller to do `separator.to_owned()` before
calling it.

This is an *expanding* mbti change: every existing caller keeps working
(String literals and `String` values coerce to `StringView`), and
StringView callers stop needing the owning copy. Internally the
implementation swaps `write_string(sep)` for `write_view(sep)`, which is
zero-copy on non-js StringBuilder.

Supersedes #3458 — `Deque::join`'s body no longer needs to work around
Iter::join's signature.

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