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?

[Rua]

Does anything else need to be done here?

[kkysen]

I think the previous way was clearer. Not sure if this commit helps.

[Rua]

The use of negated felt unclear to me in the context of checking "is null". Is it checking if "is null" is negated, or if its truthiness in the C sense is negated? With the new wording it's hopefully more clear that it's checking if "is null" is true or false.

[Rua]

I've now renamed it to is_null to hopefully make it even more obvious.