Bump version to v0.22 (#1055)

Bump version to v0.21.0 (#1010)

Bump version to v0.20 (#966)

Bump version to v0.19 (#908)

This PR bumps MMTk core to v0.19. It updates the pinned Rust version to
1.71.1, and MSRV to 1.66.

Bump version to v0.18 (#788)

Bump version to 0.17 (#760)

Bump version to v0.16 (#709)

Bump version to v0.15 (#667)

Bump version to v0.14. (#637)

Bump version to v0.13. Update Rust edition to 2021. (#616)