soroban-rpc: refactor libpreflight

[2opremio]

• Split out FFI conversions from actual preflight computations. I added a new file preflight.rs which contains the actual preflight computations, separate from the Go<->Rust ffi interfacing (which is the only thing left in lib.rs).
• Use the anyhow crate to:
  • Make error handling more readable (Result<R, Box<dyn Error>> vs Result<R>)
  • Add context to errors ( Closes soroban-rpc: Improve error reporting in preflight library #858).
• Some other minor changes, making the code more idiomatic/readable.
• Fix a couple of memory leaks in free_preflight_result()

[paulbellamy]

Now that preflight.rs is independent, it would be great to add some unit tests (which could help show me which bugs you fixed, because right now I've no idea).

[2opremio]

Now that preflight.rs is independent, it would be great to add some unit tests (which could help show me which bugs you fixed, because right now I've no idea).

Good idea, I will do it in a separate PR though, although the memory leak bugs I fixed were in lib.rs

[tsachiherman]

Few comments; overall looks great.

[2opremio]

@paulbellamy PTAL