gc: add stats and check to ms2

add a gc_sys helper variable, to use only one indirection
gc: new maybe_gc_mark gc vtable

fixes --gc ms which called the ms2! maybe_mark_and_sweep function from
fixed_allocator: allocate_new_pool_arena.

Closes GH #1143
ChangeLog for smoke-me/gc-gh1136+1143 branch
gc: fix wrong inf docs
test: add big gc stress test, to catch ms2 bug
gc: fix ms2 crash with stress/gc.1

Fixes #1143
gc: fix inf Memory_Block prologs, add -D traces

inf gc did not store string/pmc Memory_Block prologs (see pobj.h),
and Parrot_str_copy changed a Buffer_flag, i.e. the str->encoding
from the prev. string.
waste now more space in inf for these prologs.

All gc's are now pretty stable.
Fixes #1136 mostly, but not 100% yet.
gc: fix inf shim interp
[pmc] fix bignum.i_substract and i_multiply #1144

properly initialize a new temp mpf. do not the existing bn for it.
[dynpmc] rational cast FLOATVAL to double later

try to keep precision until we call the GMP function, which
can handle the enhanced precision cases better
[cage] make headerizer, fix t/codingstd/pmc_docs.t

for rational formatting