Implement the pooling instance allocator.

This commit implements the pooling instance allocator.

The allocation strategy can be set with `Config::with_allocation_strategy`.

The pooling strategy uses the pooling instance allocator to preallocate a
contiguous region of memory for instantiating modules that adhere to various
limits.

The intention of the pooling instance allocator is to reserve as much of the
host address space needed for instantiating modules ahead of time and to reuse
committed memory pages wherever possible.

crates/runtime/src/instance/allocator/pooling/unix.rs

@@ -0,0 +1,26 @@
+use crate::Mmap;
+
+pub unsafe fn make_accessible(addr: *mut u8, len: usize) -> bool {
+    region::protect(addr, len, region::Protection::READ_WRITE).is_ok()
+}
+
+pub unsafe fn decommit(addr: *mut u8, len: usize) {
+    assert_eq!(
+        libc::mmap(
+            addr as _,
+            len,
+            libc::PROT_NONE,
+            libc::MAP_PRIVATE | libc::MAP_ANON | libc::MAP_FIXED,
+            -1,
+            0,
+        ) as *mut u8,
+        addr,
+        "mmap failed to remap pages: {}",
+        std::io::Error::last_os_error()
+    );
+}
+
+pub fn create_memory_map(accessible_size: usize, mapping_size: usize) -> Result<Mmap, String> {
+    Mmap::accessible_reserved(accessible_size, mapping_size)
+        .map_err(|e| format!("failed to allocate pool memory: {}", e))
+}