unit-tests: Fix `cargo test` on 32-bit platforms 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) Differential Revision: https://phab.mercurial-scm.org/D10351