Panic when enum is used before it's declared

[T3sT3ro]

For the following:

little_endian_packets


packet TestPacket {
    a: 8,
    b: CoffeeAddition,
}

enum CoffeeAddition: 5 {
  Empty = 0,
  Other = ..
}

It fails with:

RUST_BACKTRACE=1 pdlc protocol.pdl

thread 'main' (179592) panicked at /.../.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pdl-compiler-0.4.4/src/analyzer.rs:493:23:
no entry found for key
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::option::expect_failed
   3: pdl_compiler::analyzer::Schema::new
   4: pdl_compiler::analyzer::analyze
   5: pdlc::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

It would be useful to add a message about the order of identifiers, or better yet, allow for position independent declarations.

[hchataing]

Thank you for the report, I have submitted a fix in #155