Currently, sbe_stage_builder_expression.cpp can translate the $abs agg expression into an SBE representation. To this, we will add $ceil, $exp, $floor, $ln, $log10, and $sqrt. These expressions all have identical semantics (in terms of what inputs are valid, the results of invalid inputs, etc) except for the actual numerical function that is applied, so they will be able to share a lot of code.