Prost upgrade followup

[seanlinsley]

Followup to #51, this:

• Disables the formatter for the generated protobuf file, since cargo build and cargo fmt disagree on how it should be formatted
• Replaces uses of from_i32 with try_from, since from_i32 has been deprecated:

warning: use of deprecated associated function `protobuf::ObjectType::from_i32`: Use the TryFrom<i32> implementation instead
   --> src/parse_result.rs:135:95
    |
135 |                     if let Some(protobuf::ObjectType::ObjectFunction) = protobuf::ObjectType::from_i32(s.rename_type) {
    |                                                                                               ^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default