move letification into separate class

[kroening]

The letification algorithm is the most complex aspect of the conversion to
SMT2, and is relatively independent of the rest. It may eventually have
uses elsewhere.

This is a clean move; any temptation to do improvements at the same time has been quenched, and will follow with later PRs.

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[romainbrenguier]

The class could do with a bit more documentation. But since the PR is just moving the code from smt2_conv to a new class I think it's fine.