Tighter constructor definition for Dict

[rened]

On current master the following throws unhelpful errors:

julia> Dict(1)
ERROR: BoundsError
 in call at ./dict.jl:389
 in call at dict.jl:435

julia> Dict([1,2,3])
ERROR: BoundsError
 in call at dict.jl:389
 in call at dict.jl:435

The solution is to make the type of kv in dict.jl:435:

Dict(kv) = dict_with_eltype(kv, eltype(kv))

tighter. Don't know enough about what parameter values are acceptable for a Dict constructor to do it myself...

[simonster]

I don't think the constructor can be tightened if we still want it to accept any iterator that yields pairs, but it could throw a better error message.