Refactor the rust generator

This commit performs an overhaul of the rust generator
which aims at improving the API of the generated code,
to make it more readable and usable.

The major changes are:

- packet and struct have the same generated code.
  This also means that packet derivation is also
  supported for structs now

- builder declarations are removed.
  The generator produces a unique struct for each
  packet that contains the immediate data fields,
  and fields copied from parent declarations

- constrained fields do not appear in the generated
  struct, but an accessor is provided that returns
  the constant value for the field

- the trait pdl_runtime::Packet is extended with the
  function decode() which parses the packet from an
  input slice and returns the remainder data

- TryFrom trait implementations are provided for
  all child and parent packets; these exist mostly
  for convenience and do not aim to be performant

Author: hchataing

.github/workflows/build.yml

@@ -91,6 +91,9 @@ jobs:
       - name: Run Rust generator tests
         run: pdl-compiler/tests/run_rust_generator_tests.sh
 
+      - name: Run legacy Rust generator tests
+        run: pdl-compiler/tests/run_rust_legacy_generator_tests.sh
+
       - name: Run CXX generator tests
         run: pdl-compiler/tests/run_cxx_generator_tests.sh
 
@@ -99,3 +102,6 @@ jobs:
 
       - name: Compile Rust generated files
         run: pdl-compiler/tests/compile_rust_generated_files.sh
+
+      - name: Compile legacy Rust generated files
+        run: pdl-compiler/tests/compile_rust_legacy_generated_files.sh

pdl-compiler/src/backends.rs

@@ -16,5 +16,6 @@
 
 pub mod intermediate;
 pub mod json;
+pub mod rust;
 pub mod rust_legacy;
 pub mod rust_no_allocation;