C: fix utf8 precomp len (re-utf8)

we do not have Encode nor bytes, so use the pv trick used before
to get the bytes length of an utf8 PMOP qr string.
Detected on 5.20
C: >=5.18 init utf8_tofold swash dynamically to avoid free on static errors

We need this swash with utf8 m//i, but since the &PL_utf8_tofold is empty
we need to init it somehow, to avoid free static pv2... errors (the old swash).
This does not save the folding tables into the binary.
We really need to init utf8_tofold statically (TODO)
log.modules-5.018002-nt: bad swash fix, 8 regressions
C: fix #330 with 5.18. set the PL_utf8_tofold ToCf swash statically

dynamically did not work as the existing swash got re-assigned, and the free
of the static keys failed.
Look now for the ToCf key in utf8::SWASHNEW->save and set PL_utf8_tofold to
this static swash.
Fixes issue 330 on 5.18, fixes the d676db56cf8d regressions.
module test updates (good)

and cleanup t/modules.t TODOs
1.46 release: missing module tests for 5.18/5.20