Construction of Hecke rings following Shimura

This file re-exports the Hecke ring construction, split across:

• Basic — core definitions (HeckePair, HeckeCoset, HeckeLeftCoset, decompQuot, 𝕋, HeckeModule)
• Multiplication — Shimura's heckeMultiplicity, mulMap, mulSupport, the Mul instance
• Module — smulOrbit, module action on left cosets, faithfulness
• Associativity — IsScalarTower (Shimura Prop 3.4)
• Ring — Ring (𝕋 P ℤ) instance and user-facing API
• Degree — degree ring homomorphism deg : 𝕋 P ℤ →+* ℤ (Shimura Prop 3.3)