Add the pooling-allocator feature.

This commit adds the `pooling-allocator` feature to both the `wasmtime` and
`wasmtime-runtime` crates.

The feature controls whether or not the pooling allocator implementation is
built into the runtime and exposed as a supported instance allocation strategy
in the wasmtime API.

The feature is on by default for the `wasmtime` crate.

Closes #3513.

crates/runtime/Cargo.toml

@@ -49,8 +49,11 @@ default = []
 
 async = ["wasmtime-fiber"]
 
+# Enables support for the pooling instance allocator
+pooling-allocator = []
+
 # Enables support for userfaultfd in the pooling allocator when building on Linux
-uffd = ["userfaultfd"]
+uffd = ["userfaultfd", "pooling-allocator"]
 
 # Enables trap handling using POSIX signals instead of Mach exceptions on MacOS.
 # It is useful for applications that do not bind their own exception ports and