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/src/memory.rs

@@ -364,6 +364,7 @@ impl Memory {
     }
 
     /// Returns whether or not the underlying storage of the memory is "static".
+    #[cfg(feature = "pooling-allocator")]
     pub(crate) fn is_static(&self) -> bool {
         if let Memory::Static { .. } = self {
             true