Deprecate make_and in favour of conjunction(expr, expr)

[tautschnig]

make_and does not necessarily produce an and_exprt, so its name is misleading. We already have conjunction(exprt::operandst), and will now have a variant thereof that takes exactly two operands.

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[kroening]

conjunction(a, b) behaves differently from conjunction({a, b}), which is confusing.
How about adding simplified_conjunction(a, b)?

[kroening]

Related to this, I have been considering to allow and_exprt{} (to mean true) and or_exprt{} (to mean false). Any opinions?