Make Digraph Thrift-serializable
Make Digraph Thrift-serializable

Test Plan: used it

Reviewed By: tulloch@fb.com

Subscribers: fjargsto, ntv, jhj, lesha, pamelavagata

FB internal diff: D1559030

soumith/fblualib,soumith/fblualib,raphaelamorim/fblualib,szagoruyko/fblualib,facebook/fblualib,szagoruyko/fblualib,raphaelamorim/fblualib,facebook/fblualib