Add sqrt(3)/2, -sqrt(3)/2 to arcsin, arccos

[dv-brianmickel]

Add special values to arcsin, arccos

• arcsin(sqrt(3)/2) -> pi/3
• arcsin(-sqrt(3)/2) -> -pi/3
• arccos(sqrt(3)/2) -> pi/6
• arccos(-sqrt(3)/2) -> 5pi/6

[dv-brianmickel]

75

[davidedc]

seems good, going to give it a better check tomorrow and merge

[davidedc]

thanks @dv-brianmickel !