pw_stream: Use c++20-friendly pointer operations on spans

With C++20 enabled pigweed uses std::span from the standard library.
It doesn't specify they iterator type and it is not always possible to
cast it to the pointer directly. We need to use .data() instead.

Change-Id: I4bcb6a924c0f1d4a22ef41286921bc006bc91f60
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98905
Pigweed-Auto-Submit: Dennis Kormalev <denk@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_stream/public/pw_stream/memory_stream.h b/pw_stream/public/pw_stream/memory_stream.h
index a1bbb7d..98b05c3 100644
--- a/pw_stream/public/pw_stream/memory_stream.h
+++ b/pw_stream/public/pw_stream/memory_stream.h
@@ -62,8 +62,8 @@
 
   size_t capacity() const { return dest_.size(); }
 
-  const std::byte* begin() const { return dest_.begin(); }
-  const std::byte* end() const { return dest_.begin() + position_; }
+  const std::byte* begin() const { return dest_.data(); }
+  const std::byte* end() const { return dest_.data() + position_; }
 
  private:
   size_t ConservativeLimit(LimitType type) const override {