Inexact construction of Decimal from float

[barucden]

julia> Decimal(0.1)
0.1

julia> Decimal(big(0.1))
0.1000000000000000055511151231257827021181583404541015625

This is problematic because then

julia> x = 0.1
0.1

julia> y = Decimal(x)
0.1

julia> x == y
true

julia> hash(x) == hash(y)
false

It can be improved:

julia> z = Decimal(big(0.1))
0.1000000000000000055511151231257827021181583404541015625

julia> hash(x) == hash(z)
true