[stdlib] Deprecate .unsafe_cstr_ptr()

[martinvuyk]

Deprecate .unsafe_cstr_ptr(). Closes #3601

[soraros]

Thank for the cleanup! You may also want to add fix #3601 to the description.

[yinonburgansky]

cstr_ptr != char_ptr

• cstr_ptr is null terminated
• char_ptr is not necessarily null terminated.

there could be types which need to add a null terminator.

[martinvuyk]

Hi @yinonburgansky I undestand your points, but that is what it means to work at an ABI level. Whoever uses sys.ffi knows that all operations are unsafe, it is their responsibility to know where and how pointers are coming in and out and ensure whatever "safety" can be obtained.

Beyond the philosophical debate of what safety level we should guarantee, reducing the public API surface area of unsafe operations should be a goal i.e. having less of these .unsafe...() functions. And this adds to that which I think is best in the long run.