Added support for symbolic elementary functions

[anutosh491]

This Pr implements 5 elementary symbolic functions (sin, cos, exp, log, Abs) through a macro create_symbolic_unary_macro as all of these functions take in a single argument . Hence something like the following can be accomplished.

(lf) anutosh491@spbhat68:~/lpython/lpython$ cat examples/expr2.py 
from sympy import Symbol, Abs, sin, pi, log, exp
def main0():
    x: S = Symbol('x')
    y: S = Symbol('y')
    z: S = exp(x**S(2) + pi)
    print(z)
    print(sin(z))
    print(log(z + Abs(S(-10))))

main0()

(lf) anutosh491@spbhat68:~/lpython/lpython$ lpython --backend=c --enable-symengine examples/expr2.py 
exp(x**2 + pi)
sin(exp(x**2 + pi))
log(10 + exp(x**2 + pi))

[anutosh491]

Hello @certik , This would finish the first set of TODO's related to the C backend as mentioned in the issue #1987 . I've updated the issue and pasted relevant PR's in front of each of the TODO .

[certik]

I think this looks great!

Excellent progress, thank you.

[certik]

I rebased it just in case.

[anutosh491]

Thanks a lot for the quick review Ondřej.