Run whole test suite on numba backend

[ricardoV94]

What is broken / not supported by Numba:

• float16: meta issue: fp16 support numba/numba#4402
• Sparse operations (progress in Implement Sparse Ops in Numba #1676)
• Non array/scalar global constants Reconsider handlig of constants in numba backend #1759

The plan now

• Numba becomes a default dependency
• FAST_RUN becomes an abstract mode that can use either C or Numba
  • For now it will remain C but we'll switch to Numba ASAP
  • Why Numba over C?
    • Faster
    • Easier to maintain/ extend
    • Easier to install (pip install should be safe)
• New config flag determines the backend of FAST_RUN.
  • This way downstream libraries can test if anything would break with the switch, and user can switch back if it breaks stuff for them
  • CI will run with both configs. To avoid runtime explosion it may be time to restrict floatX and fast_compile tests to a few files instead of being an orthogonal parametrization like it is now

Immediate action

• I'll be cherry picking the countless fixes into their own PRs. I need help with review as both me and @jessegrabowski are pretty much at capacity
• This PR will end up with just the test tweaks and new config flags
• We need to implement more Ops in numba mode. We'll follow up with a PR that fails on fallback to obj mode. Help needed!

[ricardoV94]

Locally I ran all the tests in tests/tensor/rewriting/test_basic.py twice (to allow caching) before and after making Numba default.
Before: 6s
After: 1m34s

Numba caching is baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaad

[ricardoV94]

After caching the test file now runs in 50s after caching vs 6s before the PR, so only 8x slower now :(