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/wasmtime/Cargo.toml

@@ -25,7 +25,7 @@ anyhow = "1.0.19"
 region = "2.2.0"
 libc = "0.2"
 cfg-if = "1.0"
-backtrace = { version = "0.3.61", optional = true }
+backtrace = { version = "0.3.61" }
 log = "0.4.8"
 wat = { version = "1.0.43", optional = true }
 serde = { version = "1.0.94", features = ["derive"] }
@@ -37,7 +37,7 @@ lazy_static = "1.4"
 rayon = { version = "1.0", optional = true }
 object = { version = "0.28", default-features = false, features = ['read_core', 'elf'] }
 async-trait = { version = "0.1.51", optional = true }
-once_cell = "1.9"
+once_cell = "1.12"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"
@@ -61,7 +61,6 @@ default = [
   'pooling-allocator',
   'memory-init-cow',
   'vtune',
-  'wasm-backtrace',
 ]
 
 # An on-by-default feature enabling runtime compilation of WebAssembly modules
@@ -106,12 +105,6 @@ posix-signals-on-macos = ["wasmtime-runtime/posix-signals-on-macos"]
 # Enabling this feature has no effect on unsupported platforms.
 memory-init-cow = ["wasmtime-runtime/memory-init-cow"]
 
-# Enables runtime support necessary to capture backtraces of WebAssembly code
-# that is running.
-#
-# This is enabled by default.
-wasm-backtrace = ["wasmtime-runtime/wasm-backtrace", "backtrace"]
-
 # Enables in-progress support for the component model. Note that this feature is
 # in-progress, buggy, and incomplete. This is primarily here for internal
 # testing purposes.