Implement calling into C

[certik]

This is supported well at the ASR level, but we need to figure out a Python syntax. Whether via the ctypes FFI, or some other syntax.

[certik]

One idea:

from ctypes import c_float, c_double
def c_sin_f32(x: c_float) -> c_float:
    pass

def c_sin_f64(x: c_double) -> c_double:
    pass

[certik]

CPython does not have 32bit floats, so we simply declare in math.py:

def sin(x: f64) -> f64:
    return c_sin_f64(x)

[certik]

I like this approach: https://code.activestate.com/recipes/576731-c-function-decorator/, that might be almost exactly what we need. An example how to use ctypes with types is, e.g., here: https://www.linkedin.com/pulse/calling-c-from-python-using-ctypes-warren-jitsing

[certik]

Initial implementation was done in #167, #171, #172. Here is a test:

• https://github.com/lcompilers/lpython/blob/4cb87bed52eb54685ba38baef307d0d0520a8beb/integration_tests/test_c_interop_01.py

It works with CPython. Now we need to get this test working with LPython.

[certik]

The LPython part was implemented in: #179, #181 and used in #184, #186. This issue is fixed. We need to extend this C interop to other types, but that will be an ongoing work as we need it.