Map Vector3i.Axis enum to builtin Vector3Axis

[Bromeon]

Other extensions which reference the integer axis type should now be able to use the builtin enum Vector3Axis.

An example that caused problems, with the GodotVoxel plugin:

info: component 'rust-std' for target 'x86_64-pc-windows-gnu' is up to date
   Compiling godot-ffi v0.1.1 (https://github.com/godot-rust/gdext?branch=master#4dc2720f)
   Compiling godot-core v0.1.1 (https://github.com/godot-rust/gdext?branch=master#4dc2720f)
error[E0433]: failed to resolve: could not find `vector3i` in `classes`
   --> C:\...\GodotVoxelSteam\voxel_generator\target\x86_64-pc-windows-gnu\debug\build\godot-core-00312bf968325b83\out\classes\voxel_blocky_model.rs:256:59
    |
256 | ...: crate::classes::vector3i::Axis, clockwise: bool,) {
    |                      ^^^^^^^^ could not find `vector3i` in `classes`
    |
note: module `crate::gen::builtin_classes::vector3i` exists but is inaccessible
   --> C:\...\GodotVoxelSteam\voxel_generator\target\x86_64-pc-windows-gnu\debug\build\godot-core-00312bf968325b83\out\builtin_classes\mod.rs:13:1
    |
13  | mod vector3i;
    | ^^^^^^^^^^^^^ not accessible

error[E0433]: failed to resolve: could not find `vector3i` in `classes`
   --> C:\...\GodotVoxelSteam\voxel_generator\target\x86_64-pc-windows-gnu\debug\build\godot-core-00312bf968325b83\out\classes\voxel_blocky_model.rs:257:49
    |
257 |             type CallSig = ((), crate::classes::vector3i::Axis, bool);
    |                                                 ^^^^^^^^ could not find `vector3i` in `classes`
    |
note: module `crate::gen::builtin_classes::vector3i` exists but is inaccessible
   --> C:\...\GodotVoxelSteam\voxel_generator\target\x86_64-pc-windows-gnu\debug\build\godot-core-00312bf968325b83\out\builtin_classes\mod.rs:13:1
    |
13  | mod vector3i;
    | ^^^^^^^^^^^^^ not accessible

For more information about this error, try `rustc --explain E0433`.
error: could not compile `godot-core` (lib) due to 2 previous errors

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-797

[greenfox1505]

This worked for my build using https://github.com/Zylann/godot_voxel.