Assertion failure in print_tyid when using a list of lists in both param/return form and with a type definition

[philpax]

I was testing out #547 with our bindings, and encountered this assert:
https://github.com/bytecodealliance/wit-bindgen/blob/main/crates/rust-lib/src/lib.rs#L245

This is the minimal repro:

default world bindings {
    import component: self.component
}

interface component {
    type value = list<tuple<>>
    type entity = list<tuple<u32, value>>

    add-components: func(entity: u64, data: entity)
    query-eval: func(q: u64) -> list<tuple<u64, list<value>>>
}

Note that the type entity is necessary; substituting its definition into add-components allows for generation to pass.

This is the resulting callstack:

% RUST_BACKTRACE=1 wit-bindgen rust wit -w main
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `"'a"`,
 right: `"'_"`', wit-bindgen-f98ccff846621481/58c130a/crates/rust-lib/src/lib.rs:245:21
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9eb3afe9ebe9c7d2b84b71002d44f4a0edac95e0/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/9eb3afe9ebe9c7d2b84b71002d44f4a0edac95e0/library/core/src/panicking.rs:64:14
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed
   4: wit_bindgen_rust_lib::RustGenerator::print_tyid
   5: wit_bindgen_rust_lib::RustGenerator::print_ty
   6: wit_bindgen_rust_lib::RustGenerator::print_tyid
   7: wit_bindgen_rust_lib::RustGenerator::print_ty
   8: wit_bindgen_rust_lib::RustGenerator::print_rust_slice
   9: wit_bindgen_rust_lib::RustGenerator::print_type_list
  10: <wit_bindgen_rust::RustWasm as wit_bindgen_core::WorldGenerator>::import_interface
  11: wit_bindgen_core::WorldGenerator::generate
  12: wit_bindgen::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

(with the latest wit-bindgen-cli installed through cargo install --git https://github.com/bytecodealliance/wit-bindgen.git wit-bindgen-cli)

[alexcrichton]

Thanks! I think #552 should fix this