Fixes https://bz.mercurial-scm.org/show_bug.cgi?id=6506
This makes IndexEntryBuilder::build, which is only used in unit tests, use
u32 or u64 instead of platform-dependent usize when packing binary data
to be used at test input.
To run Rust unit tests in 32-bit mode in a x86-64 environment, use:
rustup target add i686-unknown-linux-gnu # Once (cd rust && cargo test --target i686-unknown-linux-gnu)