Add a pooling allocator mode based on copy-on-write mappings of memfds.

As first suggested by Jan on the Zulip here [1], a cheap and effective
way to obtain copy-on-write semantics of a "backing image" for a Wasm
memory is to mmap a file with `MAP_PRIVATE`. The `memfd` mechanism
provided by the Linux kernel allows us to create anonymous,
in-memory-only files that we can use for this mapping, so we can
construct the image contents on-the-fly then effectively create a CoW
overlay. Furthermore, and importantly, `madvise(MADV_DONTNEED, ...)`
will discard the CoW overlay, returning the mapping to its original
state.

By itself this is almost enough for a very fast
instantiation-termination loop of the same image over and over,
without changing the address space mapping at all (which is
expensive). The only missing bit is how to implement
heap *growth*. But here memfds can help us again: if we create another
anonymous file and map it where the extended parts of the heap would
go, we can take advantage of the fact that a `mmap()` mapping can
be *larger than the file itself*, with accesses beyond the end
generating a `SIGBUS`, and the fact that we can cheaply resize the
file with `ftruncate`, even after a mapping exists. So we can map the
"heap extension" file once with the maximum memory-slot size and grow
the memfd itself as `memory.grow` operations occur.

The above CoW technique and heap-growth technique together allow us a
fastpath of `madvise()` and `ftruncate()` only when we re-instantiate
the same module over and over, as long as we can reuse the same
slot. This fastpath avoids all whole-process address-space locks in
the Linux kernel, which should mean it is highly scalable. It also
avoids the cost of copying data on read, as the `uffd` heap backend
does when servicing pagefaults; the kernel's own optimized CoW
logic (same as used by all file mmaps) is used instead.

[1] https://bytecodealliance.zulipchat.com/#narrow/stream/206238-general/topic/Copy.20on.20write.20based.20instance.20reuse/near/266657772

crates/runtime/src/traphandlers/unix.rs

@@ -51,9 +51,17 @@ pub unsafe fn platform_init() {
         register(&mut PREV_SIGFPE, libc::SIGFPE);
     }
 
-    // On ARM, handle Unaligned Accesses.
-    // On Darwin, guard page accesses are raised as SIGBUS.
-    if cfg!(target_arch = "arm") || cfg!(target_os = "macos") || cfg!(target_os = "freebsd") {
+    // Sometimes we need to handle SIGBUS too:
+    // - On ARM, handle Unaligned Accesses.
+    // - On Darwin, guard page accesses are raised as SIGBUS.
+    // - With the MemFD allocator, heap growth is controlled by
+    //   ftruncate'ing an mmap'd file, and so out-of-bounds accesses
+    //   are raised as SIGBUS.
+    if cfg!(target_arch = "arm")
+        || cfg!(target_os = "macos")
+        || cfg!(target_os = "freebsd")
+        || cfg!(feature = "memfd-allocator")
+    {
         register(&mut PREV_SIGBUS, libc::SIGBUS);
     }
 }