Inline nullable basic types without allocations

At the moment `Null` and `Null` used exclusively in one scope create unnecessary allocations on static platforms.

The compiler could inline those away like it does for anonymous structures:
```haxe
var a: Null = 5;
var b: Null = null;
if (a != null)
trace(a);
```
This is functionally the same as:
```haxe
var a_null: Bool = false;
var a_val: Int = 5;
var b_null: Bool = true;
var b_val: Int;
if (!a_null)
trace(a_val);
```

No files, tests, or entry points are named; begin by locating the compiler handling for nullable basic types and the existing anonymous-structure inlining. Done means single-scope Null and Null values on static platforms are represented without allocations while preserving the nullable behavior shown in the examples.

Written by the indexing model from the issue text.