debug/stracktrace: fix build when `enable-stacktrace` is disabled

[00xc]

There were a couple of errors when building with enable-stacktrace disabled (e.g. cargo b --no-default-features):

• StackUnwinder did not have a Debug derive, which caused a warning due to deny(missing_debug_implementations).
• print_stack() had different arguments depending on the feature, but it was called assuming one of those two versions from feature-independent code.
• Imports were causing warnings due to them not being used when the feature was disabled.
• Some structs were not used at all when the feature was disabled, but there were no warnings since they were (unnecessarily) declared as pub.