arrow

ARROW-2615: [Rust] Post refactor cleanup

Just a couple of trivial changes that got missed in the refactor:

- Derive Eq trait for DataType and Field (because I rely on that as a user of this library)
- ArrowPrimitiveType should NOT be implemented for strings

Author: Andy Grove <andygrove73@gmail.com>

Closes #2070 from andygrove/post_refactor_cleanup and squashes the following commits:

6b245a4e <Andy Grove> add accessor methods to ListArray
3289e3bc <Andy Grove> Update examples
236fde8e <Andy Grove> Minor post-refactor cleanup
82765f93 <Andy Grove> Merge remote-tracking branch 'upstream/master'
d1bfdca5 <Andy Grove> Merge branch 'master' of github.com:andygrove/arrow
52de6a10 <Andy Grove> Merge branch 'master' of github.com:andygrove/arrow
0e2606b2 <Andy Grove> Merge remote-tracking branch 'upstream/master'
d883da2f <Andy Grove> Merge remote-tracking branch 'upstream/master'
589ef71d <Andy Grove> Merge remote-tracking branch 'upstream/master'
bd4fbb55 <Andy Grove> Merge remote-tracking branch 'upstream/master'
9c8a10a4 <Andy Grove> Merge remote-tracking branch 'upstream/master'
05592f8c <Andy Grove> Merge remote-tracking branch 'upstream/master'
8c0e6982 <Andy Grove> Merge remote-tracking branch 'upstream/master'
31ef90ba <Andy Grove> Merge remote-tracking branch 'upstream/master'
2f87c703 <Andy Grove> Fix build - add missing import