wasmtime: add build-time option for parallel compilation (#1903)

When running in embedded environments, threads creation is sometimes
undesirable. This adds a feature to toggle wasmtime's internal thread
creation for parallel compilation.

crates/environ/Cargo.toml

@@ -20,7 +20,7 @@ cranelift-wasm = { path = "../../cranelift/wasm", version = "0.65.0", features =
 wasmparser = "0.58.0"
 lightbeam = { path = "../lightbeam", optional = true, version = "0.18.0" }
 indexmap = "1.0.2"
-rayon = "1.2.1"
+rayon = { version = "1.2.1", optional = true }
 thiserror = "1.0.4"
 directories = "2.0.1"
 sha2 = "0.8.0"
@@ -32,6 +32,7 @@ zstd = "0.5"
 toml = "0.5.5"
 file-per-thread-logger = "0.1.1"
 more-asserts = "0.2.1"
+cfg-if = "0.1.9"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"
@@ -46,5 +47,8 @@ pretty_env_logger = "0.4.0"
 filetime = "0.2.7"
 lazy_static = "1.3.0"
 
+[features]
+parallel-compilation = ["rayon"]
+
 [badges]
 maintenance = { status = "actively-developed" }