TermInterface for DynamicExpressions

[nmheim]

If desired, open a PR at DynamicExpressions.jl with the implementation of TermInterface.jl. For full compatibility with Metatheory.jl we also need to handle the metadata correctly, because dynamic Expressions need the OperatorEnum to be passed along. (I am not sure if using dynamic expressions would actually be slower than normal Exprs in the EGraph and the conversion from/to Julia Exprs is not a bottle neck for OptiFloat at all, but it would still be nice to have TI.jl)