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.

Libraries/LibDebug/Dwarf/DwarfInfo.cpp

@@ -53,7 +53,7 @@ void DwarfInfo::populate_compilation_units()
     if (m_debug_info_data.is_null())
         return;
 
-    InputMemoryStream stream(m_debug_info_data.span());
+    InputMemoryStream stream{ m_debug_info_data };
     while (!stream.eof()) {
         auto unit_offset = stream.offset();
         CompilationUnitHeader compilation_unit_header {};