Feature request: Use more idiomatic ptr null pointer APIs instead of hardcoded 0s

[64kramsystem]

When c2rust transpiles null pointers, it uses hardcoded 0s converted to the dedicated types, eg.:

static mut demoptr: *mut i8 = 0 as *const i8 as *mut i8;

this is technically correct, however, using the dedicated ptr APIs leads to more readable and idiomatic code:

static mut demoptr: *mut u8 = ptr::null_mut();

(this applies only to Sized types, though)

[kkysen]

This was already discussed in #202. There can be some subtle other differences between the two (#202 (comment)), and when refactoring, both 0 as *mut _ and ptr::null_mut() would be removed in favor of non-raw pointer APIs, so the difference usually doesn't end up mattering so much. Previous, this change could be done with the refactorer (c2rust refactor) (#202 (comment)), but that's now been deprecated as we're working on a more thorough way to lift unsafe Rust into safe Rust.