ARROW-2871: [Python] Raise when calling to_numpy() on boolean array

[wesm]

Also marks that to_numpy API is experimental, since we need to figure out what to do about nulls (ARROW-2870)

[codecov-io]

Codecov Report

Merging #2277 into master will increase coverage by 2.39%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2277      +/-   ##
==========================================
+ Coverage   84.26%   86.65%   +2.39%     
==========================================
  Files         290      234      -56     
  Lines       44299    41565    -2734     
==========================================
- Hits        37330    36020    -1310     
+ Misses       6942     5545    -1397     
+ Partials       27        0      -27

Impacted Files	Coverage Δ
python/pyarrow/tests/test_array.py	100% <100%> (ø)	⬆️
python/pyarrow/array.pxi	66.97% <100%> (+0.45%)	⬆️
go/arrow/array/booleanbuilder.go
go/arrow/array/array.go
go/arrow/type_traits_numeric.gen.go
rust/src/list.rs
go/arrow/array/binary.go
go/arrow/memory/memory_avx2_amd64.go
go/arrow/array/bufferbuilder.go
go/arrow/math/float64_sse4_amd64.go
... and 48 more

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[xhochy]

+1, LGTM