Making Texinfo XS parser the default?

[Gavin Smith]

I wondered if anybody has run Texinfo a lot with TEXINFO_XS_PARSER set
in the environment? This speeds texi2any up by about 30% but it is not
the default behaviour because I was concerned that the code hadn't
been tested very much (apart from passing Texinfo's own test suite).

Other XS extensions are used by default, e.g. the paragraph formatter.
I was wondering if it is perhaps the time to make the parser module
the default as well.

This wouldn't affect the case of when Perl extension modules were not
found to work or were disabled.