Compute Fixed Columns in several powers of 2

[georgwiese]

The idea of VadCop is that we can chose the size (i.e., number of rows) of each machine at runtime. One issue with this is that we're currently computing commitments to fixed columns of a particular size at setup time.

Instead, we should:

• Compute fixed columns for several powers of two:
  • If its namespace has a size, there should only be one size
  • If it does not, it should be computed with all the powers of two up to the maximum size of any namespace (perhaps with a lower limit like 16)
• The verifying key of CompositeBackend should contain fixed column commitments for different sizes
• The proof of CompositeBackend should indicate the size of each machine
• The verifier of CompositeBackend should select the right commitments out if the verification key.