persistent-nodemap: Fix Rust declarations for Revlog_CAPI signatures Use Rust’s `libc::ssize_t` as the closest match to C’s `Py_ssize_t`. See details in test comment. Going forward we should find a way to have such Rust declarations auto-generated from C headers at build time, or auto-checked against them in a test. Differential Revision: https://phab.mercurial-scm.org/D9901