Make Slab<T> Send if T is.
Make Slab<T> Send if T is.

Closes #90 .

jamwt/mio,jamwt/mio,fhartwig/mio,murarth/mio,geofft/mio,fuchsia-mirror/third_party-mio,carllerche/mio,dwrensha/mio,zslayton/mio,mneumann/mio,fuchsia-mirror/third_party-mio,zhangf911/mio,yuikns/mio,marjakm/mio,alexcrichton/mio,luca-barbieri/mio,hjr3/mio,zhangf911/mio,pythonesque/mio,mlalic/mio,greensnark/mio,asomers/mio,thijsc/mio,tempbottle/mio,asomers/mio,rryan/mio,yangxingpping/mio,frewsxcv/mio,mneumann/mio,danburkert/mio,tailhook/mio,polyfractal/mio,rrichardson/mio,echochamber/mio,arthurprs/mio,polyfractal/mio,dpc/mio,carllerche/mio,bottompawn/mio