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/DebugInfo.cpp

@@ -105,7 +105,7 @@ void DebugInfo::prepare_lines()
         return;
 
     auto buffer = section.wrapping_byte_buffer();
-    InputMemoryStream stream { buffer.span() };
+    InputMemoryStream stream { buffer };
 
     Vector<LineProgram::LineInfo> all_lines;
     while (!stream.eof()) {