[spec/struct] Improve introduction/overview

[ntrel]

Rename Introduction to Overview. Add subheadings: Structs, Storage, Unions.
Move struct description.
Define fields earlier, mention . operator.
Describe copying from an lvalue.
Extend & make new example runnable.
Move union grammar to Unions subheading.

Related minor changes in class.dd:
Define fields.
Instance members are always accessed with ..

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