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/cli-flags/src/lib.rs

@@ -346,9 +346,7 @@ impl CommonOptions {
             config.wasm_bulk_memory(enable);
         }
         if let Some(enable) = reference_types {
-            #[cfg(feature = "wasm-backtrace")]
             config.wasm_reference_types(enable);
-            drop(enable); // suppress unused warnings
         }
         if let Some(enable) = multi_value {
             config.wasm_multi_value(enable);