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/Cargo.toml

@@ -37,6 +37,7 @@ winapi = { version = "0.3.7", features = ["winbase", "memoryapi", "errhandlingap
 
 [target.'cfg(target_os = "linux")'.dependencies]
 userfaultfd = { version = "0.4.1", optional = true }
+memfd = { version = "0.4.1", optional = true }
 
 [build-dependencies]
 cc = "1.0"
@@ -59,3 +60,5 @@ uffd = ["userfaultfd", "pooling-allocator"]
 # It is useful for applications that do not bind their own exception ports and
 # need portable signal handling.
 posix-signals-on-macos = []
+
+memfd-allocator = ["pooling-allocator", "memfd"]