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.

Author: bobzhang

builtin/iterator.mbt

@@ -422,12 +422,12 @@ pub fn[X] Iter::flatten(self : Iter[Iter[X]]) -> Iter[X] {
 ///|
 /// Collects the elements of the iterator into a string.
 /// The old iterator `self` must not be used again after calling `join`.
-pub fn Iter::join(self : Iter[String], sep : String) -> String {
+pub fn Iter::join(self : Iter[String], sep : StringView) -> String {
   let result = StringBuilder::new()
   if self.next() is Some(x) {
     result.write_string(x)
     while self.next() is Some(x) {
-      result.write_string(sep)
+      result.write_view(sep)
       result.write_string(x)
     }
   }

builtin/pkg.generated.mbti

@@ -283,7 +283,7 @@ pub fn[X] Iter::intersperse(Self[X], X) -> Self[X]
 pub fn[X] Iter::iter(Self[X]) -> Self[X]
 #alias(iterator2)
 pub fn[X] Iter::iter2(Self[X]) -> Iter2[Int, X]
-pub fn Iter::join(Self[String], String) -> String
+pub fn Iter::join(Self[String], StringView) -> String
 #deprecated
 pub fn[X] Iter::just_run(Self[X], (X) -> IterResult) -> Unit
 pub fn[X] Iter::last(Self[X]) -> X?