Fix intersect error reported in #41096

[N5N3]

MWE

julia> A = Val{<:Union{Nothing, Val{true},Val{false}}}
Val{<:Union{Val{true}, Val{false}, Nothing}}

julia> B = Val{Val{B}} where {B}
Val{Val{B}} where B

julia> (Base.typeintersect(B, A))
Val{<:Union{Val{true}, Val{false}}}  # Val{Val{true}} before this PR

This PR makes sure env is restored between every 2 adjacent intersects during intersect_all.
The valid env changes are tracked (and merged) with a new function merge_env.
Not sure this is the correct direction to fix the problem.

Close #41096.
Edit: looks like this PR also fixes #43082.

[JeffBezanson]

Wow, this is great. Seems right to me, and other than that I go by the tests. I see one test was changed, but it's ok because the results are more conservative, so I'm happy with that. Thank you, this code is very tricky!

@nanosoldier runtests(ALL, vs = ":master")

[nanosoldier]

Your package evaluation job has completed - possible new issues were detected. A full report can be found here.

[N5N3]

@nanosoldier runtests(["CairoMakie", "CountdownNumbers", "FlameGraphs", "Folds", "PProf", "Pidfile", "ReinforcementLearningExperiments", "RetroCap", "RoundAndSwap", "StrBase"], vs = ":master")

[nanosoldier]

Your package evaluation job has completed - possible new issues were detected. A full report can be found here.