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
