Move set defaults function to its own file (#4154)

Move set defaults function to its own file (#4154)

This moves SmtEngine::setDefaults to its own file.

This design is not final. One could imagine this being a part of a "OptionsSetter" utility. I am leaving this as is until we refactor the relationship between SmtEngine and Options.

Regardless, the general file structure should be such that this method is separate from SmtEngine, since setting default options is a large task that should be addressed independently from the core of SmtEngine.

This is initial preparation towards converting the SmtEngine from Expr -> Node.

A few very minor changes were made to the code to make the separation possible.