Roadmap for the C backend user experience

[certik]

This issue is a roadmap for what we need to improve with regards to the C backend in LPython:

High Priority

• Add integration tests for the C backend #544
• Implement and test pointers (arguments)
• Implement C structs (passing as arguments, etc.)
• call other C functions that contain pointers, procedure calls, enums
• C Structs interop: Support C structs #606
• Implement and test pointers (indexing): Manipulating C pointers #546, Indexing a C array #609

Lower priority

• Import from C Header File #541
• Implement procedure calls arguments
• Syntax+semantics of p_c_pointer: Semantics of p_c_pointer and PointerToCPtr #592
• Get all strings working, including the C interop: Get C strings passing from Python to C #671
• Pointer(x) does not update x: pointer(x) doesn't update x in CPython #670

[certik]

Smaller intermediate tasks:

• Sync ASR with LFortran (https://gitlab.com/lfortran/lfortran/-/merge_requests/1785)
• Get the C backend compiling test_c_interop_03
• Get CPython running test_c_interop_03
• Callback from C
• Callbacks using CPython (test_c_interop_05 with cpython) --- not possible! One cannot build the C parts if the callbacks are missing from C. One can only provide a callback in Python and pass around a pointer --- a different use case.
• test_c_interop_04 with cpython (Implement cpython support in test_c_interop_04 #602)
• test_c_interop_03 with cpython (Implement cpython support in test_c_interop_03 #603)

[certik]

All the high priority features are implemented.

[certik]

Closing this in favor of #1575.