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ARROW-3716: [R] Missing cases for ChunkedArray conversion

*WIP* and will need some more testing of support of various chunked_array. There's also some room for less code duplication.

But I'll need that before I can follow up on the feather support.

Author: Romain Francois <romain@purrple.cat>

Closes #2928 from romainfrancois/ARROW-3716/ChunkedArray and squashes the following commits:

f56891b0a <Romain Francois> More ChunkedArray tests ✅
02de7e046 <Romain Francois> 🔀 Array__as_vector and ChunkedArray__as_vector to use the same underlying code
bdb219158 <Romain Francois> ➕ ChunkedArray<DATE32>$as_vector()
cd5a5cedf <Romain Francois> ChunkedArray<DICTIONARY>$as_vector support
b5c19b473 <Romain Francois> refactor DictionaryArray_to_Vector
889d6ef50 <Romain Francois> ➕ ChunkedArray<BOOL>$as_vector() support
85ab699a1 <Romain Francois> ➕ ChunkedArray<String>$as_vector()
1683feb64 <Romain Francois> rebase simple_Array_to_Vector and simple_ChunkedArray_to_Vector on the new simple_Array_to_Vector_Slice
fd3cca208 <Romain Francois> move ChunkedArray__as_vector to array.cpp