improve type stability of tail/front(::NamedTuple)

[ranocha]

I improved type stability to fix some invalidations when loading ChainRulesCore.jl.. This is based on the following code:

julia> using Pkg; Pkg.activate(temp=true); Pkg.add("ChainRulesCore")

julia> using SnoopCompileCore; invalidations = @snoopr(using ChainRulesCore); using SnoopCompile

julia> trees = invalidation_trees(invalidations)
...
 inserting tail(t::Tangent{<:NamedTuple{<:Any, <:Tuple{}}}) in ChainRulesCore at ~/.julia/packages/ChainRulesCore/Z4Jry/src/tangent_types/tangent.jl:110 invalidated:
...
                 9: signature Tuple{typeof(Base.tail), Any} triggered MethodInstance for Base.tail(::NamedTuple{names}) where names (335 children)