Allow writing mutators with lambdas

[alexreinking]

This follows the implementation of std::visit, using a struct to accumulate function overloads and then dispatching to the correct one at compile time (using std::is_invocable_v). The result is the ability to define mutators that capture local state. I convert a few visitors in the codebase as a means to achieve some test coverage.

For example, this is the code for expand_expr inside StorageFlattening.cpp:

// Perform all the substitutions in a scope
static Expr expand_expr(const Expr &e, const Scope<Expr> &scope) {
    Expr result = LambdaMutator{
        [&](const Variable *var, auto *mut) -> Expr {
            if (const Expr *value = scope.find(var->name)) {
                // Mutate the expression, so lets can get replaced recursively.
                Expr expr = mut->mutate(*value);
                debug(4) << "Fully expanded " << var->name << " -> " << expr << "\n";
                return expr;
            }
            return var;
        }}.mutate(e);
    debug(4) << "Expanded " << e << " into " << result << "\n";
    return result;
}

We need the pointer to the mutator as the second argument.

[alexreinking]

If we like this direction, we can make a version for visitors, too.

[alexreinking]

We can also consider adding mutate_with and mutate_with_generic helpers that immediately construct the LambdaMutator and call .mutate on it, which will simplify some call sites.

[alexreinking]

We can also consider adding mutate_with and mutate_with_generic helpers that immediately construct the LambdaMutator and call .mutate on it, which will simplify some call sites.

I went ahead and did this -- clang-format decides to add a line, but I prefer how the functional form reads.