polexpr 0.8 update

[jfbu]

Hi Karl,

It seems polexpr 0.8 which was announced at CTAN on March 30
did not make its way to texlive and thus probably somehow
got lost in the backlog post TL21 release.

Relevant: this update converts it into a "generic" package,
usable with Plain:

tex/generic/polexpr/polexpr.sty
tex/generic/polexpr/polexprcore.tex
tex/generic/polexpr/polexprexpr.tex
tex/generic/polexpr/polexprsturm.tex

the two other files are polexpr.html (documentation)
and README.md

I hesitated bothering you about this, because I might soon
release 0.8.1 anyhow, but I am taking this opportunity to point
out the change of category from LaTeX to generic area, which
may be a bit of burden to you,

(polexpr.sty despite its name is usable in Plain via \input)

Thanks for the TL21 release,

Best,

Jean-François