Module mz_adapter::explain::mir

EXPLAIN support for MIR structures.

The specialized Explain implementation for an MirRelationExpr wrapped in an Explainable newtype struct allows us to interpret more mz_repr::explain::ExplainConfig options. This is the case because attribute derivation and let normalization are defined in mz_transform and conssequently are not available for the default Explain implementation for MirRelationExpr in mz_expr.