Min string length for COW

Min string length for COW

We have two separate length thresholds for when copy-on-write kicks
in, one for when a buffer would have had to be (re)allocated
(SV_COW_THRESHOLD) and another for when there is already a large
enough buffer available (SV_COWBUF_THRESHOLD).

Benchmarking against mktables and against Test.Simple’s test suite
(see JS::Test::Simple on CPAN) run with WWW::Scripter and JE shows
that 0/1250 is the best combination, at least on 32-bit darwin.

Apparently, copying into an existing buffer is much faster than the
bookkeeping overhead of sv_force_normal_flags (which I see no way to
speed up).

I have defined these conditionally with #ifndef, so that platform-spe-
cific hints can override them with values appropriate to the platform.

Also, refactor things in sv_setsv_flags slightly to avoid using SvLEN
and SvCUR repeatedly.