Prevent TOML invalidation by pirates of T[elements...]

StaticArrays (perhaps among others) semi-pirates this method,
although they've worked hard to make it pretty innocuous.
Still, there are times when it invalidates the TOML-reading,
so best to protect this.

base/toml_parser.jl

@@ -663,7 +663,9 @@ function push!!(v::Vector, el)
         push!(v, el::T)
         return v
     elseif T === Union{}
-        return t[el]
+        out = Vector{t}(undef, 1)
+        out[1] = el
+        return out
     else
         if typeof(T) === Union
             newT = Any