v0.17: Support for more Variant types, math functions

Notable feature:

• A variety of 32- and 64-bit Math functions has been added.
• The C++ container now uses ccache to reduce compilation times.
• Making function calls on objects is now much faster than before.
• Worked around a nasty bug in ObjectDB::instantiate(). Variant destructors overwrite something in the underlying godot::Object*.

This release adds proper support and testing for:

• Vector3/3i
• Vector4/4i
• Color
• PackedInt32Array
• PackedInt64Array
• PackedVector2Array
• PackedVector3Array
• PackedColorArray
• Other remaining Variants are passed by reference, and can still be used.

What's Changed

• Measure accumulated Sandbox instantiation time by @fwsGonzo in #124
• Add math functions and corresponding system calls by @fwsGonzo in #126
• Implement 32-bit float math system calls by @fwsGonzo in #127
• Implement wrapper for Color variant type by @fwsGonzo in #128
• Various by @fwsGonzo in #129
• Implement faster calls on Object, with reference support by @fwsGonzo in #130
• Remaining packed arrays by @fwsGonzo in #131

Full Changelog: v0.16...v0.17