consolidate external code (attempt 2)

[tjhance]

See the explanation on the previous PR: #1315

This PR was merged and reverted because of some issues with traits.

This new PR attempts to match the current behavior when it comes to which trait-impls lifetime_generate generates. See the commit "add is_item_explicitly_external for legacy behavior"

[utaal]

Can you please add a brief description of the changes in the PR?

[tjhance]

Could someone do a veritas run for me?

I've been trying, but I'm having trouble installing docker.

[Chris-Hawblitzel]

Could someone do a veritas run for me?

Everything seems to work except for https://github.com/microsoft/verified-storage.git , which gives this error:

thread 'rustc' panicked at rust_verify\src\lifetime_generate.rs:2506:18:
unexpected impl ImplItemRef { id: ImplItemId { owner_id: DefId(0:519 ~ lib[dc17]::pmem::traits_t::{impl#19}::SIZE) }, ident: SIZE#0, kind: Const, span: tmp\store\storage_node\src\pmem\traits_t.rs:153:5: 153:37 (#0), trait_item_def_id: Some(DefId(0:502 ~ lib[dc17]::pmem::traits_t::ConstPmSized::SIZE)) }
stack backtrace:
   0:     0x7ffc19e70f51 - std::backtrace_rs::backtrace::dbghelp64::trace
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\..\..\backtrace\src\backtrace\dbghelp64.rs:91
   1:     0x7ffc19e70f51 - std::backtrace_rs::backtrace::trace_unsynchronized
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\..\..\backtrace\src\backtrace\mod.rs:66
   2:     0x7ffc19e70f51 - std::sys::backtrace::_print_fmt
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\sys\backtrace.rs:66
   3:     0x7ffc19e70f51 - std::sys::backtrace::impl$0::print::impl$0::fmt
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\sys\backtrace.rs:39
   4:     0x7ffc19ea2629 - core::fmt::rt::Argument::fmt
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/core\src\fmt\rt.rs:177
   5:     0x7ffc19ea2629 - core::fmt::write
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/core\src\fmt\mod.rs:1178
   6:     0x7ffc19e67117 - std::io::Write::write_fmt<std::sys::pal::windows::stdio::Stderr>
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\io\mod.rs:1823
   7:     0x7ffc19e74069 - std::panicking::default_hook::closure$1
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\panicking.rs:266
   8:     0x7ffc19e73bec - std::panicking::default_hook
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\panicking.rs:293
   9:     0x7ffc19e74a42 - std::panicking::rust_panic_with_hook
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\panicking.rs:797
  10:     0x7ffc19e74886 - std::panicking::begin_panic_handler::closure$0
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\panicking.rs:671
  11:     0x7ffc19e71a0f - std::sys::backtrace::__rust_end_short_backtrace<std::panicking::begin_panic_handler::closure_env$0,never$>
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\sys\backtrace.rs:170
  12:     0x7ffc19e74496 - std::panicking::begin_panic_handler
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\panicking.rs:662
  13:     0x7ffc1cc347d4 - core::panicking::panic_fmt
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/core\src\panicking.rs:74
  14:     0x7ff651bdea3f - <unknown>
  15:     0x7ff651be251d - <unknown>
  16:     0x7ff651902e15 - <unknown>
  17:     0x7ff651ba6bed - <unknown>
  18:     0x7ff6519c5193 - <unknown>
  19:     0x7ffc16f36c18 - _rust_alloc_error_handler
  20:     0x7ffc16f32a16 - _rust_alloc_error_handler
  21:     0x7ffc16f3c5db - _rust_alloc_error_handler
  22:     0x7ffc19e8640d - alloc::boxed::impl$48::call_once
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/alloc\src\boxed.rs:2231
  23:     0x7ffc19e8640d - alloc::boxed::impl$48::call_once
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/alloc\src\boxed.rs:2231
  24:     0x7ffc19e8640d - std::sys::pal::windows::thread::impl$0::new::thread_start
                               at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library/std\src\sys\pal\windows\thread.rs:55
  25:     0x7ffd55d9259d - BaseThreadInitThunk
  26:     0x7ffd5752af38 - RtlUserThreadStart