Add some type assertions (#831)

... to improve type stability in some situations.

Also remove two redundant dict writes: get! already stores these

src/ideal/IdealTypes.jl

@@ -12,7 +12,7 @@ mutable struct IdealSet{T <: AbstractAlgebra.NCRingElem} <: Set
  function IdealSet{T}(R::PolyRing) where T
  return get!(IdealSetID, R) do
  new(R)
- end
+ end::IdealSet{T}
  end
 end
 

src/matrix/MatrixTypes.jl

@@ -14,7 +14,7 @@ mutable struct matrix_space{T <: Nemo.RingElem} <: Set
  function matrix_space{T}(R::PolyRing, r::Int, c::Int) where T
  return get!(MatrixSpaceID, (R, r, c)) do
  new{T}(R, r, c)
- end
+ end::matrix_space{T}
  end
 end
 

src/module/ModuleTypes.jl

@@ -13,7 +13,7 @@ mutable struct FreeMod{T <: Nemo.RingElem} <: Module{T}
  function FreeMod{T}(R::PolyRing, r::Int) where T
  return get!(FreeModID, (R, r)) do
  new(R, r)
- end
+ end::FreeMod{T}
  end
 end
 
@@ -63,7 +63,7 @@ mutable struct ModuleClass{T <: Nemo.RingElem} <: Set
  function ModuleClass{T}(R::PolyRing) where T
  return get!(ModuleClassID, R) do
  new(R)
- end
+ end::ModuleClass{T}
  end
 end
 

src/number/NumberTypes.jl

@@ -35,10 +35,9 @@ mutable struct Integers <: Ring
  return get!(IntegersID, :ZZ) do
  ptr = libSingular.nInitChar(libSingular.n_Z, Ptr{Nothing}(0))
  d = new(ptr, 0)
- IntegersID[:ZZ] = d
  finalizer(_Ring_finalizer, d)
  return d
- end
+ end::Integers
  end
 end
 
@@ -78,10 +77,9 @@ mutable struct Rationals <: Field
  return get!(RationalsID, :QQ) do
  ptr = libSingular.nInitChar(libSingular.n_Q, Ptr{Nothing}(0))
  d = new(ptr, 0)
- RationalsID[:QQ] = d
  finalizer(_Ring_finalizer, d)
  return d
- end
+ end::Rationals
  end
 end