[m-rev.] for review: implement the rest of the extension changes proposal

[Julien Fischer]

On Thu, 17 Aug 2023, Zoltan Somogyi wrote:

> For review by anyone, and for tryout by Julien on Windows.

Bootechecked on Windows with both GCC and MSVC. I have tried
it out with --use-subdirs / --use-grade-subdirs in the 
hlc.gc / java / csharp / reg.gc.decldebug.stseg trades and
not found any issues.

Julien.