Add quantization utils and modules

[kevinchern]

• spin-to-bit and bit-to-spin conversion utils
• binarization methods with straight-through gradients
• nothing fancy about these. Some are as simple as def f(x): return 2*x-1 but they're still useful as named functions instead of writing 2*x-1 everywhere

TODOs:

• document
• test
• release note
• wrap with store_config