Correct hashing of unitful values

Reposted from https://github.com/JuliaData/DataFrames.jl/issues/2486.

Currently we have:
```
julia> isequal(1u"d", 24u"hr")
true

julia> hash(1u"d"), hash(24u"hr")
(0xc0114c5d5037b321, 0x34c298a544e80792)
```
and `hash` requires that if two values pass `isequal` test they must have the same hash.

CC @tomyun

Start by locating the implementations of hash and isequal for unitful values; the issue provides a reproducing example but names no files or tests. Ensure values that pass isequal, including 1u"d" and 24u"hr", produce the same hash, then add or run a regression check for that invariant.

Written by the indexing model from the issue text.