Implement round built-in function

[namannimmo10]

We can implement a generic round once overload is supported in LPython (#187), but for now this works for float arguments.

TODO:

• fix the case when the compile-time value is 0.5 (Outputs with Python and C++ are different: 0 and 1 respectively)

[certik]

Looks good so far.

[namannimmo10]

Gives garbage values for the 6 print statements:

1075230278
1090940434
1078307718
1071644672
1078575105
1071644672

I think that's because we are calling other built-in functions in our current runtime implementation.

[certik]

I usually get the function (round in this case) working first in a simple script. Then put it into the runtime library.

[namannimmo10]

OK, this is ready for now; let me know if you have any suggestions or I'll merge this. Generic versions of these built-in functions will be added soon.

[certik]

I think this looks great!