Add an LWE noise propagation model

[j2kun]

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[j2kun]

TODO

• infer max noise as best I can
• test

[j2kun]

Putting this on hold for a few more days while I work on other priorities. Notes to self:

• Last thing I did was replace the "optional" version of the variance lattice with a three-state variant: uninitialized (not a ciphertext value), set (a known value), and unbounded (a pessimistic fixedpoint).
• The tests aren't quite raising errors how I expect, for example in validate_noise_errors.mlir it's not raising an error because the function's block argument isn't being set to the right value. It seems it should either be unbounded or lwe.add should have special handling for uninitialized error (currently says "unknown" when it should be more specific).
• I did not end up overriding visitNonControlFlowArguments because I think it's not necessary, but I may be wrong on that.