[wip] basis extension part of keyswitch lowering

[j2kun]

[j2kun]

Some TODOs:

• Implement the final linalg.reduce to sum along the k dimension
• Support NTT of vector<RNS> type
• Clean up PolynomialToModArith (remove/improve commonConversionInfo to use best practices for type conversion)
• Add lit tests

For after this ticket:

• Lower convert_basis
• Discuss what we can do to ease the task of implementing new high-level primitives (e.g., a new bootstrap implementation); maybe this means we need a kernel language or an embedded DSL.
• Do an e2e functional test of the keyswitch lowering overall