Introduce crumbles - keeping track of dirt in userspace

[ureeves]

crumbles is intended to solve the problem of mapping WASM memories onto virtual address space.

It is built to support tracking dirty pages and creating memory snapshots to which it is possible to revert to. This is required if we want to remove re-execution in piecrust, as well as allowing for the removal of diffing in the storage process of state commits.

Since it uses system calls that only exist on *nix systems it can only build on Linux and MacOS. The tracking of dirty pages is done slightly differently on Linux and MacOS, and it might be useful to test it actively for different combinations of platform and architecture. It is important to note that using this would not be a regression for piecrust since it currently only builds on *nix as well.

See also: #253
See also: #254

[fed-franz]

LGTM!

Nice lib and love the name :)

Couple of nits in the comments.

Also: since you define MEM_SIZE and PAGE_SIZE, it might be worth using those in the description, instead of assuming is 4Gib/64Kib. This would turn useful if you plan to release the lib with those value as input parameters.

[ureeves]

LGTM!

Nice lib and love the name :)

Couple of nits in the comments.

Also: since you define MEM_SIZE and PAGE_SIZE, it might be worth using those in the description, instead of assuming is 4Gib/64Kib. This would turn useful if you plan to release the lib with those value as input parameters.

I haven't made those public, so referring to them in the docs would be wrong. I'll make them public, since it would be a breaking change to modify them anyway.

[moCello]

LGTM, just some tinitiny suggestions but nothing major.
Is rustfmt.toml supposed to be deleted though?