Revert allowing non-literal bitstype immutables

As pointed out by @simonster, this ends up `eval`-ing any call written into the HDF5 file as a type parameter and is a security threat.

src/JLD.jl

@@ -751,7 +751,7 @@ is_valid_type_ex(s::Symbol) = true
 is_valid_type_ex(s::QuoteNode) = true
 is_valid_type_ex{T}(::T) = isbits(T)
 is_valid_type_ex(e::Expr) = ((e.head == :curly || e.head == :tuple || e.head == :.) && all(map(is_valid_type_ex, e.args))) ||
-                            (e.head == :call && isa(e.args[1], Symbol))
+                            (e.head == :call && (e.args[1] == :Union || e.args[1] == :TypeVar))
 
 # Work around https://github.com/JuliaLang/julia/issues/8226
 const _typedict = Dict{String,Type}()

test/jld.jl

@@ -191,7 +191,6 @@ Sbig = "A test string "^1000
 type BitsParams{x}; end
 bitsparamfloat = BitsParams{1.0}()
 bitsparambool  = BitsParams{true}()
-bitsparamimmut = BitsParams{PaddingTest(1,2)}()
 bitsparamsymbol = BitsParams{:x}()
 
 iseq(x,y) = isequal(x,y)
@@ -370,7 +369,6 @@ for compress in (true,false)
     @write fid Sbig
     @write fid bitsparamfloat
     @write fid bitsparambool
-    @write fid bitsparamimmut
     @write fid bitsparamsymbol
 
     # Make sure we can create groups (i.e., use HDF5 features)
@@ -488,7 +486,6 @@ for compress in (true,false)
         @check fidr Sbig
         @check fidr bitsparamfloat
         @check fidr bitsparambool
-        @check fidr bitsparamimmut
         @check fidr bitsparamsymbol
 
         x1 = read(fidr, "group1/x")