x/mobile/bind: support slices of structs

[kevmo314]

Slices of pointers to bound structs, such as []*S, are now supported
as parameters, results and struct fields. They cross the language
barrier as slices of reference numbers, so Java sees S[] and
Objective-C sees NSArray<S*>*.

Unlike byte slices, which are passed by reference, the elements are
converted one by one; mutating the slice on one side does not affect
the other. A nil slice maps to a null array and an empty slice to an
empty array, and a nil element maps to null in Java and NSNull in
Objective-C.

Fixes golang/go#13445

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Patch Set 1:

(1 comment)

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Message from Kevin Wang:

Patch Set 2:

(1 comment)

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[gopherbot]

This PR (HEAD: 19dd1fc) has been imported to Gerrit for code review.

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