[rust] Initial commit of Rust bindings

SEC-88

Test: Manually tested:
- new roll_boringssl.go
- compiled/tested rust/boringssl-sys
- compiled //garnet/public/rust/crates/mundane-crypto, which
  depends on these bindings

Change-Id: I2804a88d19e7755ff43e17ca75e4180e73417158
6 files changed