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/instance/allocator.rs

@@ -23,8 +23,10 @@ use wasmtime_environ::{
     SignatureIndex, TableInitializer, TrapCode, VMOffsets, WasmType, WASM_PAGE_SIZE,
 };
 
+#[cfg(feature = "pooling-allocator")]
 mod pooling;
 
+#[cfg(feature = "pooling-allocator")]
 pub use self::pooling::{
     InstanceLimits, ModuleLimits, PoolingAllocationStrategy, PoolingInstanceAllocator,
 };