Added support for comparing symbolic expressions

[anutosh491]

While I was discussing comparsion of class types (Pow etc) with @Thirumalai-Shaktivel we realized that currently we aren't supporting comparing basic variables. Hence this is an implementation for that. Now we can do the following

(lf) anutosh491@spbhat68:~/lpython/lpython$ cat examples/expr2.py 
from lpython import S
from sympy import Symbol, pi, sin

def main0():
    x: S = pi
    y: S = pi
    print(x == pi)
    print(x == y)
    print(pi == y)
    print(pi == pi)
    print(x != sin(x))
    print(pi != sin(pi))

(lf) anutosh491@spbhat68:~/lpython/lpython$ lpython --enable-symengine examples/expr2.py 
True
True
True
True
True
True

This should essentially help us with the following (https://github.com/sympy/sympy/blob/368eb0f483f2b4cd84dc0de3edc7abdc876d3d46/sympy/series/gruntz.py#L257)

[Thirumalai-Shaktivel]

Perfect, let's add some tests.

[anutosh491]

I've added some simple tests in the file where we are testing out binary operators. Hopefully this works.