Unexpected payload size for non byte aligned payload

[Martin Geisler]

The following PDL file crashes backends::rust::generate, even though it passes through the analyzer:

little_endian_packets

struct Foo {
  ts_sec: 20,
  ts_usec: 32,
  _size_(_payload_): 32,
  _payload_,
}

Stack trace:

thread '<unnamed>' panicked at 'Unexpected payload size for non byte aligned payload', /home/mgeisler/src/pdl/src/backends/rust/parser.rs:526:17
stack backtrace:
   0: begin_panic_handler
             at /rustc/b2b34bd83192c3d16c88655158f7d8d612513e88/library/std/src/panicking.rs:593:5
   1: panic_fmt
             at /rustc/b2b34bd83192c3d16c88655158f7d8d612513e88/library/core/src/panicking.rs:67:14
   2: pdl_compiler::backends::rust::parser::FieldParser::add_payload_field
             at ./src/backends/rust/parser.rs:526:17
   3: pdl_compiler::backends::rust::parser::FieldParser::add
             at ./src/backends/rust/parser.rs:77:17
   4: pdl_compiler::backends::rust::generate_data_struct
             at ./src/backends/rust.rs:236:9
   5: pdl_compiler::backends::rust::generate_struct_decl
             at ./src/backends/rust.rs:696:38
   6: pdl_compiler::backends::rust::generate_decl
             at ./src/backends/rust.rs:979:13
   7: pdl_compiler::backends::rust::generate::{{closure}}
             at ./src/backends/rust.rs:998:53