Add extension for FixedSizeArrays (collect_structarray initializer)

[qwjyh]

Adding small package extension for FixedSizeArrays to provide initializer for collect_structarray to collect an iterator into a StructArray of FixedSizeArray.
I used a hack code in __init__ of the extension to export the new initializer under StructArrays module.
(The name of the initializer is still so long.)

[aplavin]

I wonder, why does it need to live in StructArrays? The new code doesn't use any of the internals (doesn't use any code from StructArrays at all, for that matter), seems like it can just as well be independent.
Note that we don't have custom initializers for StaticArrays to be used with collect_structarray here, even though there's already an extension for StaticArrays.

[qwjyh]

I thought that StructArrayInitializer is internal API since it is not documented and that collect_structarray is a bit difficult to use as its document is not so detailed.
So is it better to add some docs to collect_structarray and StructArrayInitializer with examples on FixedSizeArray instead?

[aplavin]

Sorry, I must've missed the StructArrayInitializer part originally :)
Why is this needed? Looking at the docs for collect_structarray, they say:

The user can optionally pass a `initializer`, that is to say
a function `(S, d) -> v` that associates to a type and a size an array of eltype `S`
and size `d`. By default `initializer` returns a `StructArray` of `Array` but custom array types
may be used.

Can initializer be just a function (S, d) -> similar(FixedSizeArrayDefault{S, length(d)}, d)?

[qwjyh]

Can initializer be just a function (S, d) -> similar(FixedSizeArrayDefault{S, length(d)}, d)?

I think this is totally fine so I'm closing this PR.