Implementation of lerp without a function requires repeating one of the arguments, which is not ideal. To avoid that, add a new function to the driver namespace. In addition, provide a function for clamping between 0 and 1 to support easy clamped lerp, and a smoothstep function from GLSL that is somewhat related. The function implementations are added to a new bl_math module. As an aside, add the round function and two-argument log to the pylike expression subset. Differential Revision: https://developer.blender.org/D8205