move GenericMemory-related definitions to the main file (JuliaArrays#86)

They're not `FixedSizeArray`-specific, might be useful for
`FixedSizeBitArray`.

Author: nsajko

src/FixedSizeArray.jl

@@ -28,12 +28,6 @@ end
 const FixedSizeVector{T} = FixedSizeArray{T,1}
 const FixedSizeMatrix{T} = FixedSizeArray{T,2}
 
-const default_underlying_storage_type = (@isdefined Memory) ? Memory : Vector
-
-const optional_memory = (@isdefined Memory) ? (Memory,) : ()
-const optional_atomic_memory = (@isdefined AtomicMemory) ? (AtomicMemory,) : ()
-const optional_generic_memory = (@isdefined GenericMemory) ? (GenericMemory,) : ()
-
 function FixedSizeArray{T,N,V}(::UndefInitializer, size::NTuple{N,Int}) where {T,N,V}
     FixedSizeArray{T,N,V}(Internal(), V(undef, checked_dims(size))::V, size)
 end

src/FixedSizeArrays.jl

@@ -1,5 +1,11 @@
 module FixedSizeArrays
 
+const default_underlying_storage_type = (@isdefined Memory) ? Memory : Vector
+
+const optional_memory = (@isdefined Memory) ? (Memory,) : ()
+const optional_atomic_memory = (@isdefined AtomicMemory) ? (AtomicMemory,) : ()
+const optional_generic_memory = (@isdefined GenericMemory) ? (GenericMemory,) : ()
+
 include("FixedSizeArray.jl")
 
 if isdefined(Base, :dataids) && (Base.dataids isa Function)