Add warning on impossible nil checks

It's common, especially when porting CLJ code, to forget to make some type nullable. The error is generally only caught at runtime. Here I propose to add a warning every time the compiler can detect a nil check will never be true, this will help spot the error at compile time.

This includes inlines of nil? and some? but also boolean contexts where the type is not bool and which is thus only a nil check (we already have specialized code for that).

Start by locating the compiler paths for nil? and some? inlines and the existing specialized handling for boolean contexts where the type is not bool. Trace how nullable and non-nullable types are represented during these checks. Done means the compiler warns whenever it can prove a nil check cannot be true, covering all three cases described in the issue.