hab-sup panics when pkg_export tries to export struct

[jaym]

This is ascii order dependent:

# default.toml
[service]
port=1111

[foo]
foo="foo"
bar="bar"

pkg_exports=(
[http-port]=service.port
[foo]=foo
)

does not work work

pkg_exports=(
[foo]=foo
[http-port]=service.port
)

does not work work

hab-sup dies with:

thread 'main' panicked at 'Struct should serialize to bytes: ValueAfterTable', libcore/result.rs:945:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.

================

The following works:

# default.toml
[service]
port=1111

[moo]
foo="foo"
bar="bar"

pkg_exports=(
[http-port]=service.port
[moo]=moo
)

pkg_exports=(
[moo]=moo
[http-port]=service.port
)

[jamesc]

Full stacktrace on panic

stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:477
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:391
   6: rust_begin_unwind
             at libstd/panicking.rs:326
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   8: core::result::unwrap_failed
             at libcore/macros.rs:26
   9: <core::result::Result<T, E>>::expect
             at libcore/result.rs:835
  10: habitat_butterfly::rumor::service::Service::new::{{closure}}
             at ./components/butterfly/src/rumor/service.rs:116
  11: <core::option::Option<T>>::map
             at libcore/option.rs:414
  12: habitat_butterfly::rumor::service::Service::new
             at ./components/butterfly/src/rumor/service.rs:115
  13: habitat_sup::manager::service::Service::to_rumor
             at components/sup/src/manager/service/mod.rs:671
  14: habitat_sup::manager::Manager::gossip_latest_service_rumor
             at components/sup/src/manager/mod.rs:1473
  15: habitat_sup::manager::Manager::add_service
             at components/sup/src/manager/mod.rs:657
  16: habitat_sup::manager::Manager::start_initial_services_from_spec_watcher
             at components/sup/src/manager/mod.rs:1695
  17: habitat_sup::manager::Manager::run
             at components/sup/src/manager/mod.rs:697
  18: hab_sup::sub_run
             at components/sup/src/main.rs:190
  19: hab_sup::start
             at components/sup/src/main.rs:140
  20: hab_sup::main
             at components/sup/src/main.rs:84
  21: std::rt::lang_start::{{closure}}
             at libstd/rt.rs:74
  22: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  23: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:103
  24: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  25: std::rt::lang_start
             at libstd/rt.rs:74
  26: main
  27: __libc_start_main
  28: _start

Looks like @fnichol might have worried about this at time of writing: https://github.com/habitat-sh/habitat/blob/master/components/butterfly/src/rumor/service.rs#L113-L117

// TODO FN: Can we really expect this all the time, should we return a `Result<Self>`
 // in this constructor?
cfg: cfg
  .map(|v| toml::ser::to_vec(v).expect("Struct should serialize to bytes"))
                .unwrap_or_default(),

[jamesc]

Fixed by #5987