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/fuzzing/src/generators.rs

@@ -213,6 +213,7 @@ pub struct WasmtimeConfig {
     codegen: CodegenSettings,
     padding_between_functions: Option<u16>,
     generate_address_map: bool,
+    wasm_backtraces: bool,
 }
 
 /// Configuration for linear memories in Wasmtime.
@@ -387,6 +388,7 @@ impl Config {
             .wasm_multi_memory(self.module_config.config.max_memories > 1)
             .wasm_simd(self.module_config.config.simd_enabled)
             .wasm_memory64(self.module_config.config.memory64_enabled)
+            .wasm_backtrace(self.wasmtime.wasm_backtraces)
             .cranelift_nan_canonicalization(self.wasmtime.canonicalize_nans)
             .cranelift_opt_level(self.wasmtime.opt_level.to_wasmtime())
             .consume_fuel(self.wasmtime.consume_fuel)