Transform trigonometric functions into complex exponentials : trig2exp

### 5.21.20 Transform trigonometric functions into complex exponentials : trig2exp

trig2exp takes as argument an expression containing trigonometric functions.
trig2exp rewrites the trigonometric functions with complex exponentials (without linearization).
Input :

trig2exp(tan(x))

Output :

((exp((i)*x))`^`2-1)/((i)*((exp((i)*x))`^`2+1))

Input :

trig2exp(sin(x))

Output :

(exp((i)*x)-1/(exp((i)*x)))/(2*i)