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ARROW-4853: [Rust] Array slice doesn't work on ListArray and StructArray

This fixes a few issues related to Array offset and slicing:

1. The invariant check in ListArray::from() is no longer valid for the case
  of offset. This removes the check.
2. Both `Array::is_valid` and `Array::is_null` should take into account of offset.
3. `StructArray::len()` should return `data.len()` instead of the length of the
  first child array. In the case of slicing, we'll not modify child array data so
  the previous approach will return wrong result.

Author: Chao Sun <sunchao@apache.org>

Closes #3972 from sunchao/ARROW-4853 and squashes the following commits:

299d6a43 <Chao Sun> Fix StructBuilder reuse
c8633467 <Chao Sun> Improve unit tests
78e8e880 <Chao Sun> Fix format
e0fed916 <Chao Sun> Fix StructArray::column
8278c786 <Chao Sun> Fix StructArray::From<Vec<(Field, ArrayRef)>>
c7814331 <Chao Sun> Fix StructBuilder length & unit tests
99af8d50 <Chao Sun> ARROW-4853:  Array slice doesn't work on ListArray and StructArray