rust: simply use TryInto to convert slice to array
Since our rust module depends on TryInto, there's no point to avoid using it.
While rewriting copy_into_array(), I noticed CPython interface doesn't check
the length of the p1/p2 values, which is marked as TODO.
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: