WIP: Make hash(::CategoricalValue) faster by pre-computing hashes

[nalimilan]

This optimization only works for the one-argument hash() method: when a hash
code is passed, the hash needs to be recomputed anyway. This can still be useful
for hashing DataTables columns.

[coveralls]

Coverage decreased (-0.3%) to 90.963% when pulling 3d3aa36 on nl/hash into b88769b on master.

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by -0.35%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage    91.3%   90.96%   -0.35%     
==========================================
  Files           9        9              
  Lines         506      509       +3     
==========================================
+ Hits          462      463       +1     
- Misses         44       46       +2

Impacted Files	Coverage Δ
src/pool.jl	95.83% <0%> (-1.01%)	❌
src/typedefs.jl	100% <100%> (ø)	✅
src/value.jl	68.29% <100%> (-1.71%)	❌
src/buildfields.jl	100% <100%> (ø)	✅

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[nalimilan]

x-ref JuliaData/DataFrames.jl#1565.