Prevent erroneous code like vector<fbstring>{{"this", "that"}} from compiling

Prevent erroneous code like vector<fbstring>{{"this", "that"}} from compiling

Summary: Someone debugged a runtime crash and traced it back to code like: `vector<fbstring>{{"this", "that"}}`. Presumably the user thought they were passing an initializer list of strings to the vector constructor. Instead, they constructed a single fbstring with two char pointers pointing into //different// strings. With the appropriate fbstring constructors, we can flag this as invalid at compile-time.

Reviewed By: yfeldblum

Differential Revision: D3927397

fbshipit-source-id: a5f335073fb55bbb703a23f06874238cbdb5d91a