RT 43789: "in memory" files don't call STORE

RT 43789: "in memory" files don't call STORE

The code in PerlIO-scalar that implements the open $fh, '>' \$buffer
feature did not, apart from accidentally, support get/set magic and thus
tied buffers. This patch remedies that: mostly by just blindly sprinkling
SvGETMAGIC/SvSETMAGIC about, rather than doing any deep analysis and
understanding of the code. One main change I did was to add a
PerlIOScalar_read() function, rather than rely on the default behaviour
(which implements it in terms of PerlIOScalar_get_ptr() etc), since that
approach had a tendency to call FETCH multiple times