Unique Symbol Generation

[LSLeary]

The project I want to test with io-sim uses Data.Unique, so I wrote this.

P.s. I'm not yet familiar with the library, let alone its internals—do let me know if I've written something silly.

[coot]

Thank you for your contributions. A few minor comments.

[LSLeary]

Re the trace events, giving them the actual IOSimUnique s would require adding a type parameter to SimEventType or making the constructor existential in s. I'm not sure what the best choice is here, so for now it just holds the contained Integer.

[coot]

We can trace Integer rather than IOSimUnique, this will avoid the s parameter.

[coot]

Thank @LSLeary, the PR looks good to me. Just some minor things. Please squash the commits and I'll merge it.