Use foreach and sizeHint in mapAccumulate

[cheeseng]

mapAccumulate traversed via a manual
val it = iterator; while (it.hasNext) { ... it.next() ... } loop,
even though nothing about the method needs iterator-specific
capability — no early exit, and every element is visited
unconditionally.

Switch to foreach, which dispatches to each collection's own native
traversal (e.g. List walking cons cells directly, index-based loops
for array-backed types) instead of allocating an Iterator and paying
two virtual calls (hasNext/next) per element.

This is safe for mapAccumulate specifically because the state (S) is
threaded sequentially through calls to f in traversal order, and
foreach/iterator are guaranteed to visit a given collection's
elements in the same order as each other — so the switch doesn't
change which elements state gets threaded through in which order.

Also add b.sizeHint(this): mapAccumulate always produces exactly one
B per input element, so unlike some other methods in this file where
a sizeHint would only be a guess, here the result size is known
exactly in advance and the builder can size itself accordingly.

No behavior changes.