Feature: Assume arbitrary number of formulae at once

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assume(f1, f2, f3, ...)

Earlier, they had to each call assume once.

The other versions of assume had to be removed due to currently unresolved overloading issues. Ideally, we would like to have a second version accepting Iterable[Formula], but that case can be handled with the current Formula*. The String version is unused and causes an overload error, so removed it. The string should be explicitly cast to Formula if one wishes to assume it.