initiate hhbc_hhas.rs, a bytecode printing module

initiate hhbc_hhas.rs, a bytecode printing module

Summary:
- hhbc_hhas.ml prints bytecode from `Hhas` and aast, Ocaml version converts it to a list of string. List of string is like to avoid string concatenation. In Rust, a `Write` trait is introduced, it is meant be an abstraction over various channels can take string. An implementation for `std::fmt::Write` is added, therefore `String` implements `Write`,
- a set of write utilities are included to catch common patterns,
- in Compile.rs, a `String` is passed to `print_program`, later it will be changed to std output or file handlers.

Reviewed By: losvald

Differential Revision: D19458379

fbshipit-source-id: c6094e053b3a273c056648efe064766daa78d825