Merge v2ray.com/ext into v2ray.com/core

[VictoriaRaymond]

Due to the uncertainty of Go module, it might be easier to develop in a mono-repo. All source code will be merged into core/. There will no user-facing changes.

[VictoriaRaymond]

For developers, here is the detailed change of this merge:

• ext/assert: Removed completely.
• ext/bazel: Moved to core/infra/bazel
• ext/tools/conf: Moved to core/infra/conf
• ext/tools/control: Moved to core/infra/control
• ext/tools/vprotogen: Moved to core/infra/vprotogen

The ext repo will remain unchanged. Your code may still depend on ext. But we highly recommend you to update your references.