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/Cargo.toml

@@ -22,7 +22,7 @@ indexmap = "1.0.2"
 thiserror = "1.0.4"
 more-asserts = "0.2.1"
 cfg-if = "1.0"
-backtrace = { version = "0.3.61", optional = true }
+backtrace = { version = "0.3.61" }
 rand = "0.8.3"
 anyhow = "1.0.38"
 memfd = { version = "0.4.1", optional = true }
@@ -44,7 +44,6 @@ maintenance = { status = "actively-developed" }
 
 [features]
 memory-init-cow = ['memfd']
-wasm-backtrace = ["backtrace"]
 
 async = ["wasmtime-fiber"]