[analysis] Let Flat lattice take multiple types

[tlively]

Previously, one could approximate a Flat lattice whose elements could
have multiple types by creating a Flat lattice of a variant type.
However, this would produce elements that were variants of variants,
wasting space on an extra discriminant. To make this use case more
efficient and ergonomic, support taking multiple type parameters in
Flat. The multiple type parameters all become part of the element
variant type. To handle the case where types are repeated, also add
element accessors templatized on the type index.

[kripken]

I can guess at what this line does, but where in the docs is it described? I can't seem to find that (the variant of visit with two objects after the lambda)

[tlively]

std::visit is fun because it can take an arbitrary number of std::variant arguments as long as the visitor functions can take that same number of parameters. See the Variants&&... values variadic parameter given in https://en.cppreference.com/w/cpp/utility/variant/visit2.html.