AK: Rename span() to bytes() when appropriate.

I originally defined the bytes() method for the String class, because it
made it obvious that it's a span of bytes instead of span of characters.

This commit makes this more consistent by defining a bytes() method when
the type of the span is known to be u8.

Additionaly, the cast operator to Bytes is overloaded for ByteBuffer and
such.

AK/Vector.h

@@ -243,6 +243,9 @@ public:
         return !(*this == other);
     }
 
+    operator Span<T>() { return span(); }
+    operator Span<const T>() const { return span(); }
+
     bool contains_slow(const T& value) const
     {
         for (size_t i = 0; i < size(); ++i) {