new intrinsics functions

[Lugatod]

Let's add new intrinsics functions on floats : abs and cbrt

max and min will aslo be very useful :

max(a:float,b:float) max(a:vector(float)) max(a:vector(float),b:vector(float))

Even though max and min will be more efficient on index expressions, there are also very useful to build new apps.

[stephenchouca]

Raymond (the new SuperUROP in our group) is currently working on adding support for the intrinsics proposed in #43 to the Simit compiler.