Missing module aliasing for Time_ns in Sec. 6.3

[erikyao]

Hi RWO writers,

Hope this message finds you well.

I found that module Time_ns is firstly introduced in Sec. 6.3, Reusing Field Names, but it's until Sec. 7.2, Combining Records and Variants, that you make the alias module Time_ns = Core.Time_ns.

I think the aliasing is necessary for all code in Chapter 6 that are dependent on Time_ns, and it's better to be introduced in the beginning of Sec. 6.3.

Best regards