Only allow bv2nat/int2bv with BV and integer logic (#4118)

Only allow bv2nat/int2bv with BV and integer logic (#4118)

CVC4 supports `bv2nat` and `int2bv` to convert bit-vectors to/from
integers. Those operators are not standard. This commit only enables
those operators when parsing is non-strict and both bit-vectors and
integers are enabled in the logic. To achieve this, the commit
simplifies the handling of logics in the parser: Instead of defining a
separate `Logic` enum in the `Smt2` class, we simply use `LogicInfo`
directly.