Summary:
[Folly] Fix `small_vector::insert` with non-random-access iterator arguments.

Closes https://github.com/facebook/folly/issues/1322.

Reviewed By: ot, terrelln

Differential Revision: D20150453

fbshipit-source-id: 184e748dcbb7cb60318b5f0a3104d9aeb8352d7b