Add alternate way to initialize ExpressionForest
Add alternate way to initialize ExpressionForest

Instead of instructing ExpressionForest to create a set of trees, it is
now possible to simply give it a set of trees to incorporate.

Note that doing so is dangerous as there are currently few checks in
place to ensure that the resulting structure will be sane.

Signed-off-by: Teodor-Dumitru Ene <2853baffc346a14b7885b2637a7ab7a41c2198d9@gmail.com>

tdene/synth_opt_adders