windows: add console input record event member functions

[aymanbagabas]

Events in INPUT_RECORD are defined as union with multiple
members. Since Go doesn't have a notion of unions, we need
to decode the input event into their respective types. Here,
we use member functions that match the union type names to get
the respective event type.

This is based on #227

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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