hash(::Decimal) fails

[altre]

Thanks for this package! Calling hash on a Decimal fails in julia 1.5.3:

julia> p =parse(Decimal, "0.1")
Decimal(0, 1, -1)

julia> hash(p)
ERROR: MethodError: no method matching decompose(::Decimal)

Which effectively prevents many other important methods, such as unique:

julia> unique([p,p])
ERROR: MethodError: no method matching decompose(::Decimal)
Closest candidates are:
  decompose(::BigFloat) at hashing2.jl:195
  decompose(::Float64) at hashing2.jl:184
  decompose(::Float32) at hashing2.jl:173

[jmkuhn]

Duplicate of #35.