Replace IXML with SXML (for Cloud)

[sandraros]

Big PR related to #632.

• Classic abap2xlsx classes

  • Refer to ZCL/ZIF_EXCEL_XML... instead of CL/IF_IXML...
  • Use of data references with CREATE_OSTREAM_(X|C)STREAM methods

• XML

  • ZCL/ZIF_EXCEL_XML(...) - Same (...) object type names as IXML classes and interface parts used by abap2xlsx (same method and parameter names, except it's a short list of what was used by abap2xlsx till now) - Parameter types refer to ZCL/ZIF_EXCEL_XML... instead of CL/IF_IXML... but run SXML. - implementations don't contain any reference to CL/IF_IXML...
    • contains many test classes to make sure that SXML and IXML results into identical XML
      • The tests run IXML only if ZCL_EXCEL_IXML exists (part of NOT_CLOUD package)
  • ZCL_EXCEL_XML_DOCUMENT
    • Same as CL_XML_DOCUMENT parts used by abap2xlsx

• NOT_CLOUD

  • ZCL_EXCEL_IXML - contains references to CL/IF_IXML... (it's why it's in NOT_CLOUD)
    • used by ZCL_EXCEL_XML test classes to run abap2xlsx with IXML (if IXML is installed)

Potential issues:

• Performance. Nothing optimized.
• SXML is more strict than IXML and less permissive on positioning attributes, namespace declarations, etc.
• OO design: Abuse of private instantiation/all friends.
• Probably many I didn't see...

[AndreaBorgia-Abo]

On 731:

Additional note: with the current "main" (both library and demos), checker reports some differences in 37, 38, and 15_01. The same checks on S4H (official SAP HANA appliance) bring up no findings. Disabling the strict comparison only works within the PR branch, so no chance to check on 731.

[sandraros]

Sorry, as Lars said, in the end IXML doesn't need to be replaced with SXML, it's only few classes CL_IXML to be replaced with CL_IXML_CORE, etc., all IF_IXML_* remain unchanged, so I remove my PR. It has to be recoded differently. Thanks for looking at it anyway! 👍