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

.github/workflows/main.yml

@@ -136,6 +136,7 @@ jobs:
     - run: cargo check -p wasmtime --no-default-features --features async
     - run: cargo check -p wasmtime --no-default-features --features uffd
     - run: cargo check -p wasmtime --no-default-features --features pooling-allocator
+    - run: cargo check -p wasmtime --no-default-features --features memfd-allocator
     - run: cargo check -p wasmtime --no-default-features --features cranelift
     - run: cargo check -p wasmtime --no-default-features --features cranelift,wat,async,cache
 
@@ -310,11 +311,13 @@ jobs:
       env:
         RUST_BACKTRACE: 1
 
-    # Test uffd functionality on Linux
+    # Test Linux-specific functionality
     - run: |
         cargo test --features uffd -p wasmtime-runtime instance::allocator::pooling
         cargo test --features uffd -p wasmtime-cli pooling_allocator
         cargo test --features uffd -p wasmtime-cli wast::Cranelift
+        cargo test --features memfd-allocator -p wasmtime-cli pooling_allocator
+        cargo test --features memfd-allocator -p wasmtime-cli wast::Cranelift
       if: matrix.os == 'ubuntu-latest' && matrix.target == ''
       env:
         RUST_BACKTRACE: 1