Improve perf in substitution

When looking at perf/compiler/T12545 I see this kind of thing as the top entries in ticky allocation

    Entries      Alloc    Alloc'd  Non-void Arguments      STG Name
--------------------------------------------------------------------------------
    3137531  386383024          0   2 LL                   GHC.Utils.Misc.$wsplitAtList{v rg1m} (fun)
    7490472  364368232          0   3 i.M                  Data.IntMap.Internal.$winsert{v ruzA} (fun)
    4785911  353204632          0   2 >L                   GHC.Base.map{v 01X} (fun)
    4418053  251655200          0   2 LL                   GHC.List.zip{v 0x} (fun)
    4976632   89297760          0   2 >L                   GHC.Utils.Misc.strictMap{v r4ga} (fun)
    1085849   60019872          0   2 SL                   GHC.Tc.Utils.Zonk.zonkTcTypesToTypesX{v rn} (fun)

I think the splitAtList stuff comes from splitting argument lists in Coercion. And zip is, I think, from zipTyEnv, from substTyWith, also invoked from Coercion.

There is low hanging fruit here:

• splitListAt generates extra allocation. In the common case where there are no "leftover" arguments, it's all wasted work.
• We zip together tyvars and types, but immediately take it apart in mkVarEnv.

See also #18541 which identifies other low-hanging fruit.

Some attention here might pay dividends.