make backtrace collection a Config field rather than a cargo feature (#4183)

* sorta working in runtime

* wasmtime-runtime: get rid of wasm-backtrace feature

* wasmtime: factor to make backtraces recording optional. not configurable yet

* get rid of wasm-backtrace features

* trap tests: now a Trap optionally contains backtrace

* eliminate wasm-backtrace feature

* code review fixes

* ci: no more wasm-backtrace feature

* c_api: backtraces always enabled

* config: unwind required by backtraces and ref types

* plumbed

* test that disabling backtraces works

* code review comments

* fuzzing generator: wasm_backtrace is a runtime config now

* doc fix

crates/runtime/src/externref.rs

@@ -704,7 +704,6 @@ impl VMExternRefActivationsTable {
         }
     }
 
-    #[cfg_attr(not(feature = "wasm-backtrace"), allow(dead_code))]
     fn insert_precise_stack_root(
         precise_stack_roots: &mut HashSet<VMExternRefWithTraits>,
         root: NonNull<VMExternData>,
@@ -867,7 +866,6 @@ impl<T> std::ops::DerefMut for DebugOnly<T> {
 ///
 /// Additionally, you must have registered the stack maps for every Wasm module
 /// that has frames on the stack with the given `stack_maps_registry`.
-#[cfg_attr(not(feature = "wasm-backtrace"), allow(unused_mut, unused_variables))]
 pub unsafe fn gc(
     module_info_lookup: &dyn ModuleInfoLookup,
     externref_activations_table: &mut VMExternRefActivationsTable,
@@ -895,7 +893,6 @@ pub unsafe fn gc(
         None => {
             if cfg!(debug_assertions) {
                 // Assert that there aren't any Wasm frames on the stack.
-                #[cfg(feature = "wasm-backtrace")]
                 backtrace::trace(|frame| {
                     assert!(module_info_lookup.lookup(frame.ip() as usize).is_none());
                     true
@@ -937,7 +934,6 @@ pub unsafe fn gc(
         });
     }
 
-    #[cfg(feature = "wasm-backtrace")]
     backtrace::trace(|frame| {
         let pc = frame.ip() as usize;
         let sp = frame.sp() as usize;