Implement a 2-arg Base.hash() instead of 1-arg

[JamesWrigley]

This prevents a ton of invalidations:

 inserting hash(x::Measures.Length{U}) where U @ Measures ~/.julia/packages/Measures/PKOxJ/src/length.jl:19 invalidated:                                                                                                                       
   backedges: 1: superseding hash(x) @ Base hashing.jl:28 with MethodInstance for hash(::Any) (1202 children)

Discovered when looking into JuliaLang/IJulia.jl#1208.

CC @t-bltg, this is a dependency of Plots.jl.