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ARROW-5949: [Rust] Implement Dictionary Array

@nevi-me and @andygrove

I've been working on the structure of the dictionary array getting the structure back into the spirit of the original design. The DictionaryArray is now typed by the key native type and could be typed by the value array type as otherwise, we can't implement value() in any meaningful way.

I've extended ArrowNativeType for use as an index (to/from usize) and added a value iterator which simplifies iteration over arrays of all kinds.

Closes #6095 from andy-thomason/ARROW-5949 and squashes the following commits:

aebc6258f <andy-thomason> Format the code.
2365cb306 <andy-thomason> Fix Andy G's issues - thanks.
b23f3624c <andy-thomason> Hack src/flight to pass tests.
f212bcfbe <andy-thomason> Rebase to current master.
0f1bbb3f8 <andy-thomason> Fix merge error.
adfa037e3 <andy-thomason> Format code.
03895e64d <andy-thomason> Add ArrowDictionaryKeyType subtype.
f36cccca8 <andy-thomason> Remove option for nulls in values.
2351677e4 <andy-thomason> Change unwraps to expects
d9da53429 <andy-thomason> Format code!
5c40b5f40 <andy-thomason> Add FromIterator<&str/Option<&str>> to DictionaryArray.
5f9e1fc5c <andy-thomason> Add StringDictionaryBuilder with null-zero schema.
b970c79d6 <andy-thomason> Format the code.
80a05f0fc <andy-thomason> Rebase latest.
36fbd6b67 <andy-thomason> Remove keys slice and replace with iterator.
be0041b74 <andy-thomason> Fix comment typo.
5c975ab90 <andy-thomason> Fix code formatting.
29b43bc5d <andy-thomason> Fix and test Debug for DictionaryArray
02bc6d36e <andy-thomason> Fix key issue with primitive dictionary builder.
69d678180 <andy-thomason> Remove tuple from Dictionary datatype.
1e0e84923 <andy-thomason> Add missing case to Display for ArrowError
4e54ef305 <Renjie Liu> ARROW-7312:  Implement std::error::Error for ArrowError.
1a95bac81 <Neville Dipale> add dictionary_by_field to stream reader
9d8f29045 <andy-thomason> Change to original layout and add primitive builder.
bd27cfbce <andy-thomason> Format
48689be04 <andy-thomason> Specialise the Dictionary Array by key type.
77f8a33f5 <andy-thomason> Unreachable in create_dictionary_array.
044aea677 <andy-thomason> Comment change.
c9d6dd539 <Andy Thomson> Update rust/arrow/src/array/array.rs
c19969c6d <Andy Thomson> Update rust/arrow/src/array/array.rs
f7213bb77 <Andy Thomson> Update rust/arrow/src/array/array.rs
f3416e0fc <Andy Thomson> Update rust/arrow/src/util/integration_util.rs
1fbd7cc44 <Andy Thomson> Update rust/arrow/src/ipc/file/reader.rs
497e16809 <andy-thomason> fix schema test
ee278d55b <andy-thomason> Rebase master

Lead-authored-by: andy-thomason <andy.thomason@genomicsplc.com>
Co-authored-by: Andy Thomson <andy@andythomason.com>
Co-authored-by: Neville Dipale <nevilledips@gmail.com>
Co-authored-by: Renjie Liu <liurenjie2008@gmail.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>