cleanup includes in folly/Format.h
cleanup includes in folly/Format.h

Summary:
`folly/Format.h` pulls in `folly/small_vector.h` and
`folly/FBVector.h`, which (according to @oleksandr's analysis)
are the most expensive includes for multiple fbcode projects.

Test Plan:
fbconfig -r folly/test && fbmake runtests_opt -j32

Used contbuild to catch and fix issues with transitive includes.

Reviewed By: ott@fb.com

Subscribers: net-systems@, trunkagent, atlas2-eng@, salex, ntv, jhj, ves, grangier, hphp-diffs@, search-fbcode-diffs@, reedriley, tingy, fbcode-common-diffs@, davejwatson, andrewcox, njormrod, folly-diffs@, yfeldblum, haijunz, tudort, chalfant, oleksandr

FB internal diff: D2198904

Tasks: 7520298

Signature: t1:2198904:1435543451:d77d33dc453071bc6aab9ce4df5dd7365acf2d76

peterlei/fboss,sonoble/fboss,sonoble/fboss,raphaelamorim/fboss,biddyweb/fboss,neuhausler/fboss,peterlei/fboss,peterlei/fboss,neuhausler/fboss,raphaelamorim/fboss,peterlei/fboss,neuhausler/fboss,sonoble/fboss,raphaelamorim/fboss,biddyweb/fboss,neuhausler/fboss,biddyweb/fboss