v0.6.0

SQL Anywhere 0.6.0 — signed, self-describing extensions

The loadable-extension ABI can now describe itself, so a version mismatch is
something an extension detects rather than crashes on, and every extension this
project publishes ships with a signed manifest saying exactly what it is.

BREAKING: the extension thunk's layout changed. iVersion is now the first member
of sqlanywhere_api_routines, which moves close_hook, so extensions built against
0.5.2 or earlier crash when loaded into 0.6.0. This affects the prebuilt
crsqlite archives on every release from v0.3.1 through v0.5.2; use the v0.6.0
builds instead. The SQLite C API and the database file format are unchanged.

Also in this release: SHA256SUMS, MANIFEST.json and a detached Ed25519 signature
on extension releases, with cargo xtask sign-extensions / verify-extensions; a
fix for a null-thunk dereference in the vendored cr-sqlite extension; and the
removal of a second workflow that published an unsigned archive outside the
manifest.