handle unions of tuples better

[vtjnash]

It seems that many of our tuple-type manipulation functions in Base (eltype, tuple_type_cons, tuple_type_tail, tuple_type_head) assume that Tuples can't be put into Unions. This is rather silly.

Many of these also seem to be unnecessarily marked as @pure, when it may be possible to write them more succinctly in a way that inference can optimize much more effectively than just simply being pure:

tuple_type_head(TT::Type) = fieldtype(TT::Type{<:Tuple}, 1)

[vtjnash]

most of these are deprecated now and written better