Usability issues #1802

These are relatively simple usability issues that we should fix. High priority (blocker):

• Bug C backend: Convert Cptr to c_p_pointer #1890
• Need syntax for passing CPtrs to structs and arrays of structs #1799
• Add a test for taking a CPtr pointer of a struct (dataclass) from Python #1787
• casting between struct array and CPtr causes lpython to segfault despite generating valid C code #1798 - Add lower_bounds in CPtrToPointer #1822
• Need a way to create a structure in LPython, pass a pointer to C #1801
• Bug Union init - C backend + Cpython + Lpython tests #1915
• Bug C backend: Cannot import Union  #1914

Medium priority:

• integration_tests/expr_16.py fails in LLVM #1859
• Casting from f64 to u64 does not work #1867

Lower priority:

• Fix printing of unsigned integers #1896
• Hide the "numpy" module unless --with-intrinsic-mods is used #1893
• Add a test for testing a pointer for nullptr #1781 - Support nullptr testing #1805
• Disallow parameter (argument) redefinition #1784
• Support allocatable arrays #1785 (C: Implement allocatable #1829, Support allocatable in lpython #1815)
• Require structs to be initialized before using #1788
• array of structures does not seem to work properly #1789
• Enforce dtype for empty() #1790
• Implement --fast tests in integration_tests #1793
• @ccallable: emit a header file #1797
• Struct initialization segfaults with name arguments #1800
• Implement "import A; A.something()" #1809
• c_p_pointer does not support pointers to structs with enum fields #1818
• pointer(.) of struct array causes CPython to yield error, "Type dtype('O') not implemented" #1817
• NotImplementedError: Type cannot be None #1819
• Give an error message if function parameter is declared twice #1849