Separate XS module for Texinfo::Convert::Texinfo

* tta/perl/Texinfo/Convert/TexinfoNonXS.pm (convert_to_texinfo),
tta/perl/Texinfo/Convert/TexinfoXS.pm (convert_to_texinfo),
tta/perl/Texinfo/Convert/Texinfo.pm (BEGIN, convert_to_texinfoNonXS),
tta/perl/XSTexinfo/convert/ConvertToTexinfoXS.xs
(plain_texinfo_convert_tree), tta/perl/XSTexinfo/convert/ConvertXS.xs,
tta/perl/XSTexinfo/Makefile.am (xs_LTLIBRARIES, CLEANFILES)
(EXTRA_DIST, ConvertToTexinfoXS_la_*): add TexinfoNonXS.pm and
TexinfoXS.pm with the convert_to_texinfo implementation without and
with XS.  Move the plain_texinfo_convert_tree XS interface function to
a new XS file used only for the Texinfo::Convert::Texinfo module,
ConvertToTexinfoXS.xs.  Add a BEGIN block in Texinfo.pm to load the
Perl modules and XS module and remove the function override.  Rename
_convert_to_texinfo as convert_to_texinfoNonXS and leave it in the
main Texinfo.pm module as it may be called by both TexinfoNonXS.pm and
TexinfoXS.pm.

* tta/perl/Texinfo/Convert/PlainTexinfo.pm (convert_tree): adapt for
change in Texinfo::Convert::Texinfo.
10 files changed