Port signed_source.ml to Rust
Summary: Port [signed_source.ml](https://www.internalfb.com/code/fbsource/%
5B9cc4202e36a9%5D/fbcode/hphp/hack/src/utils/signed_source.ml) (which itself is a port of [signed_source.py](https://www.internalfb.com/code/fbsource/[
424a7ea148d69a0a6f1ba32ca5f950ce657155f6]/tools/signedsource_lib/signedsource.py)) to Rust. Replace the ad-hoc signing implementation in hh_codegen.
Reviewed By: edwinsmith
Differential Revision:
D38163503
fbshipit-source-id:
e4dc4f96b43e6c34e37c331bfbd2f04f50b6c128