In exact analogy to how Mathlib supports additive homomorphisms (AddHom) φ: G →+ H between (G:Type) [Add G] and (H:Type) [Add H], and multiplicative homomorphisms (MulHom) φ: G →* H between (G:Type) [Mul G] and (H:Type) [Mul H], expand Magma.lean to support magma homomorphisms (MagmaHom) φ: G →∘ H between (G:Type) [Magma G] and (H:Type) [Magma H], with all the basic API copied over from Mathlib. Similarly create support for MagmaEquiv analogously to AddEquiv and MulEquiv. This will help support #104 and #36.