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.