ARM support and build system refactoring

[rafie]

• Dockerfile.arm for ARM (arm64v8/arm32v8) cross-build
• /build/docker/Makefile for multi-arch Docker image construction
• Relocated RedisModulesSDK submodule into deps/
• Added deps/readies submodule for infrastructure falicities
• Build system facilities: /build/mk
• system-setup.py for prerequisites installation
• Relocated tests: src/tests/ => /tests
• Build binary files now go into bin/$os-$arch-$variant
• CircleCI config.yml refactoring

[mnunberg]

Had an overview talk about this change with Rafi; I'd appreciate that at least for this stage, we branch out the build stuff to a different repository and have a script which downloads/checks out the readies/paella/etc. stuff - so there is less 'baggage' in the actual code itself (we can vendor it later on). Other than that, it looks good

[rafie]

I prefer to have it inline for the inter-module integration phase for the sense of agility (like, letting different modules behave a little differently until the whole thing is stabilized). As a matter of fact, I'm trying to avoid walking the submodule route at the moment, and explicit downloading will require me to manage versions on the readies repo, which is also kind of improductive. I say, let's take it in, and when we decide how to go about infrastructure code in general (i.e. RLTools etc), we'll align.