add edge detection to signal library

[LFSaw]

As proposed by @sletz over at #44 and in the slack channel, I'd like to suggest adding functions for edge detection to the signal library.

my proposal would be this naming scheme:

strictRising(x) = x > x';     // 1 for strictly rising, 0 otherwise
strictFalling(x) =  x < x';  // 1 for strictly falling, 0 otherwise
changed(x) = x != x';	  // 1 if signal changes, 0 otherwise
falling(x) = x <= x';   // 1 for falling or unchanged, 0 otherwise
rising(x) = x >= x'; // 1 for rising or unchanged, 0 otherwise
unchanged(x) = x == x';     // 1 for unchanged, 0 otherwise 

I understand that @sletz would like to have edge or front in the names, however, I do not see the need for "edge" (the change can be infinitesimally small for the functions to trigger)...

other naming schemes were (in order of appearance above):

• upfront / rising_edge
• downfront / falling_edge
• edge / front
• inv_upfront / inv_rising_edge
• inv_downfront / inv_falling_edge
• inv_edge / inv_front

[sletz]

see #98.