Implemented symbolic constant E

[anutosh491]

I've made this pull request as a helping PR for #2386

[anutosh491]

We've tested out the constant pi enough, E would behave just as same as pi. Not sure if I should add some test for it. All operations that could be done on pi can be replicated for E, so all tests that pass for expressions involving pi would also work for E

[certik]

Definitely add a test for E. That's to ensure that everything actually works and that it will not get broken in the future.

[anutosh491]

Done. I've added a minimalistic test function as of now which checks overall use of E.

[anutosh491]

Not really sure why the LPython CI / LPython CI (3.10, ubuntu-latest) test fails . I am not able to replicate it locally !

EDIT: Re-running the CI makes it pass.

[certik]

Thanks! What was the failure at the CI?

[anutosh491]

I think so 1 test (test_complex_02.py) failed abruptly with the C backend with the fast flag. Thirumalai told me that he has seen the failure before and it occurs sometimes in the CI. He adviced me to re-run the tests!

[certik]

It's a flaky test then. Can you report the error into an issue with a log of the failure? We need to keep track of this, so that we can fix it.