stage2: rework AIR memory layout

This commit changes the AIR file and the documentation of the memory
layout. The actual work of modifying the surrounding code (in Sema and
codegen) is not yet done.

src/codegen.zig

@@ -2,7 +2,7 @@ const std = @import("std");
 const mem = std.mem;
 const math = std.math;
 const assert = std.debug.assert;
-const ir = @import("air.zig");
+const Air = @import("Air.zig");
 const Type = @import("type.zig").Type;
 const Value = @import("value.zig").Value;
 const TypedValue = @import("TypedValue.zig");