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ARROW-3665: [Rust] Implement StructArrayBuilder

This implements `StructArrayBuilder` which can be used to build struct
arrays. There are some trickness in terms of being able to store child
builders of different types. A natural way is to box them into
`ArrayBuilder` trait and store in a vector. But, this makes it
impossible to cast them into specific type in `field_builder()`.

To solve the above issue, this maintains two references to each input
builder instance, one in `Box<Any>` type and another in
`Box<ArrayBuilder>` type. The former is used for casting mentioned
above, while the latter is used for calling general methods on an
builder, such as `len()` and `finish()`.

To enable this, this also changed `ArrayBuilder::finish` to return a
`ArrayRef` instead of a specific array. The old `finish` method is
implemented on each specific array builder, so one can obtain, say,
`Int32Array` from a `Int32ArrayBuilder`.

Author: Chao Sun <sunchao@apache.org>

Closes #3276 from sunchao/ARROW-3665 and squashes the following commits:

8fa3b61 <Chao Sun> Rename to_any to into_box_any
8aac785 <Chao Sun> Add append
227f3b2 <Chao Sun> Add from_schema
0e8e669 <Chao Sun> Fix rustfmt issues
5f59518 <Chao Sun> ARROW-3665:  Implement StructArrayBuilder