transpile: Factor out more pointer-related functions

[Rua]

This factors out functions for two things: casts involving pointers, and null checking. In the case of == and != operators in a non-conditional context (in operators.rs), this also adds special handling for regular pointers, where only function pointers were specially handled previously. This means that

int b = ptr == NULL;

is now translated with the is_null method as well, instead of with the == operator and core::ptr::null.

[kkysen]

By the way, when you do refactoring PRs like this, can you try to split out the pure moves vs. the semantic changes more carefully/fine-grained? It makes it much harder to review when mostly moves are mixed with some other changes (although I'm not sure if there are better ways of viewing the diffs; I tried --color-moved but it doesn't really work usually).

[Rua]

By the way, when you do refactoring PRs like this, can you try to split out the pure moves vs. the semantic changes more carefully/fine-grained?

How is this now?