Handle message marks on non-64-bit platforms

[ulyssa]

This fixes the issue with running the iamb tests on i686 seen in void-linux/void-packages#49652. I initially tried to do this in modalkit, but ended up deciding that usize is really the right type for Cursor and not u64. I've instead done the conversion here by using usize::MAX to make the hash predictably convertible.

[ulyssa]

Also, writing down how to run the tests w/ i686 in case I need to do it again in the future:

% sudo apt-get install crossbuild-essential-i386
% CC_i686_unknown_linux_gnu=i686-linux-gnu-gcc CARGO_TARGET_I686_UNKNOWN_LINUX_GNU_LINKER=i686-linux-gnu-gcc \
    cargo test --target i686-unknown-linux-gnu