Some reflection/tactics updates

[mtzguido]

• Pat_Var now has a sealed sort. As the type is no longer in the bv, it was difficult to write functions to traverse a term and accumulate variables with their types. Now it's as easy as before given that we use Tac.
• Pat_Wild (in reflection) and Pat_wild (in the compiler) are gone, Pat_Var is used instead.
• Tv_Unknown is now considered a legit return value for inspect/inspect_ln, the terms that cannot be inspected return Tv_Unsupp. Also Tv_Unsupp cannot be packed.