pw_span: Switch from pw::span to std::span

Replace pw::span with std::span and "pw_span/span.h" with <span>
throughout the codebase.

Change-Id: Ib1fa873168b6093794e861611d750fcad6285d6c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/12801
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_allocator/freelist_heap.cc b/pw_allocator/freelist_heap.cc
index 7966bb8..dfaba56 100644
--- a/pw_allocator/freelist_heap.cc
+++ b/pw_allocator/freelist_heap.cc
@@ -18,7 +18,7 @@
 
 namespace pw::allocator {
 
-FreeListHeap::FreeListHeap(span<std::byte> region, FreeList& freelist)
+FreeListHeap::FreeListHeap(std::span<std::byte> region, FreeList& freelist)
     : freelist_(freelist) {
   Block* block;
   Block::Init(region, &block);