[RFC/WIP] More NullableArrays compatibility

[alyst]

Add NullableArrays.jl requirement and implement a few of its methods (including the novel ones proposed by JuliaStats/NullableArrays.jl#155)

• unsafe_getindex/value_notnull()
• [unsafe_]isnull()
• padnull[!]()
• reftype(A::[Nullable]CategoricalArray) to get the type of the references of A

[nalimilan]

@DavidGold Do you think padnull! is really needed? Couldn't this be a method of resize!, with an additional before::Bool argument?

[coveralls]

Coverage decreased (-1.4%) to 89.883% when pulling 072a84f on alyst:extend_api into b88769b on JuliaData:master.

[coveralls]

Coverage decreased (-1.4%) to 89.883% when pulling 072a84f on alyst:extend_api into b88769b on JuliaData:master.

[codecov-io]

Codecov Report

Merging #29 into master will decrease coverage by 1.19%.
The diff coverage is 8.33%.

@@            Coverage Diff            @@
##           master      #29     +/-   ##
=========================================
- Coverage   92.46%   91.27%   -1.2%     
=========================================
  Files           9        9             
  Lines         690      699      +9     
=========================================
  Hits          638      638             
- Misses         52       61      +9

Impacted Files	Coverage Δ
src/nullablearray.jl	62.85% <0%> (-8.12%)	⬇️
src/array.jl	91.83% <25%> (-0.38%)	⬇️

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[coveralls]

Coverage decreased (-1.5%) to 90.706% when pulling 070a0a5 on alyst:extend_api into 0b2aa3a on JuliaData:master.

[nalimilan]

No longer applies now that we've moved to Nulls.