Turn FinGenAbGroupElem into a non-mutable struct

This reduce allocations and increases speed of code using
these elements.

src/GrpAb/Elem.jl

@@ -44,16 +44,6 @@ end
 #
 ################################################################################
 
-# This destroy's the input. If you don't want this, use A(::ZZMatrix)
-
-function FinGenAbGroupElem(A::FinGenAbGroup, a::ZZMatrix)
-  assure_reduced!(A, a)
-  z = FinGenAbGroupElem()
-  z.parent = A
-  z.coeff = a
-  return z
-end
-
 function reduce_mod_snf!(a::ZZMatrix, v::Vector{ZZRingElem})
   GC.@preserve a begin
     for i = 1:length(v)

src/HeckeTypes.jl

@@ -1828,11 +1828,16 @@ abstract type GrpAbElem <: AbstractAlgebra.AdditiveGroupElem end
 
 end
 
-mutable struct FinGenAbGroupElem <: GrpAbElem
+struct FinGenAbGroupElem <: GrpAbElem
   parent::FinGenAbGroup
   coeff::ZZMatrix
 
-  FinGenAbGroupElem() = new()
+  # This destroy's the input. If you don't want this, use A(::ZZMatrix)
+  function FinGenAbGroupElem(A::FinGenAbGroup, a::ZZMatrix)
+    assure_reduced!(A, a)
+    return new(A, a)
+  end
+
 end
 
 ################################################################################