[trace] Add orchard.trace

[alexander-yakushev]

Wanted to do this for a while. clojure.tools.trace has a bit ugly output for my taste and also adds a dependency for something simple enough that can live within Orchard. The API is compatible or almost compatible with clojure.tools.trace, so swapping one for another would be trivial.

My initial implementation of orchard.trace had pprint-like wrapping support and also configurable "spaciousness", but I decided to remove them for now for the sake of simplicity. This is something that could be added in the future if deemed necessary.

Output example:

(orchard.trace-test.sample-ns/fibo 5)
│ 
│ (orchard.trace-test.sample-ns/fibo 3)
│ │ 
│ │ (orchard.trace-test.sample-ns/fibo 1)
│ │ │ 
│ │ └─→ 1
│ │ 
│ │ (orchard.trace-test.sample-ns/fibo 2)
│ │ │ 
│ │ │ (orchard.trace-test.sample-ns/fibo 0)
│ │ │ │ 
│ │ │ └─→ 1
│ │ │ 
│ │ │ (orchard.trace-test.sample-ns/fibo 1)
│ │ │ │ 
│ │ │ └─→ 1
│ │ │ 
│ │ └─→ 2
│ │ 
│ └─→ 3
│ 
│ (orchard.trace-test.sample-ns/fibo 4)
│ │ 
│ │ (orchard.trace-test.sample-ns/fibo 2)
│ │ │ 
│ │ │ (orchard.trace-test.sample-ns/fibo 0)
│ │ │ │ 
│ │ │ └─→ 1
│ │ │ 
│ │ │ (orchard.trace-test.sample-ns/fibo 1)
│ │ │ │ 
│ │ │ └─→ 1
│ │ │ 
│ │ └─→ 2
│ │ 
│ │ (orchard.trace-test.sample-ns/fibo 3)
│ │ │ 
│ │ │ (orchard.trace-test.sample-ns/fibo 1)
│ │ │ │ 
│ │ │ └─→ 1
│ │ │ 
│ │ │ (orchard.trace-test.sample-ns/fibo 2)
│ │ │ │ 
│ │ │ │ (orchard.trace-test.sample-ns/fibo 0)
│ │ │ │ │ 
│ │ │ │ └─→ 1
│ │ │ │ 
│ │ │ │ (orchard.trace-test.sample-ns/fibo 1)
│ │ │ │ │ 
│ │ │ │ └─→ 1
│ │ │ │ 
│ │ │ └─→ 2
│ │ │ 
│ │ └─→ 3
│ │ 
│ └─→ 5
│ 
└─→ 8

[bbatsov]

Looks good to me. Great work!