Expose specialized versions of AHasher through specialized-hashers feature
          #156
        
          
      
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This adds a
specialized-hashersfeature which exposes the existing specialized hasher implementations that were under the nightly-only specialization feature for manual use. To support this, also addsBuildAHasherU64,BuildAHasherFixed, andBuildAHasherStrtypes that can be used in places that expectBuildHashers and which leverage existing infrastructure to create these specialized versions of hashers.