v0.16: Zig support, unit tests, typed arguments

• Initial Zig scripting support was added. It lacks a proper API but all the foundation is in place up to and including build containers.
• Unit tests were added, allowing us to not accidentally introduce regressions.
• A new typed primitives mode has been added and made default, requiring a rebuild of all programs. The new mode uses registers instead of Variant where possible, and is quite a bit faster as a result.
• Improved support for packed arrays in the C++ API. Packed arrays now have a wrapper class with proper methods.

What's Changed

• Add new low-latency mode that uses primitive types for certain VM call arguments by @fwsGonzo in #108
• Add fast-path using GDnative Variant for certain types by @fwsGonzo in #109
• Unit tests by @fwsGonzo in #107
• Implement low-latency mode as default by @fwsGonzo in #110
• docs: add fwsGonzo as a contributor for code by @allcontributors in #115
• docs: add fire as a contributor for code, research, and test by @allcontributors in #116
• docs: add Ughuuu as a contributor for code by @allcontributors in #117
• Initial support for Zig scripting by @fwsGonzo in #118
• Implement a tiny API for Zig compilation by @fwsGonzo in #119
• add icons by @Ughuuu in #120
• Add support for Packed Arrays in the C++ API by @fwsGonzo in #121
• Add Zig container to Docker GA by @fwsGonzo in #122

Full Changelog: v0.15...v0.16