Function `ports` is not type stable.

Consider the following two functions:

```
using Catlab.WiringDiagrams

wd = singleton_diagram(UntypedUWD, 1000000);

function collect_ports_1(wd)
ps = Int[]
for i in ports(wd)
push!(ps, i)
end
ps
end

function collect_ports_2(wd)
ps = Int[]
for i in ports(wd)::UnitRange{Int}
push!(ps, i)
end
ps
end
```

Then, on my machine,
```
collect_ports_1(wd)
@time collect_ports_1(wd);
```
outputs `0.051219 seconds (3.00 M allocations: 70.801 MiB)`.

On the other hand,
```
collect_ports_2(wd)
@time collect_ports_2(wd);
```
outputs `0.004928 seconds (17 allocations: 9.782 MiB)`.