Add benchmarks for tree creation

[alice-i-cecile]

    > Hmm... basically looks like performance isn't affected much at all. That benchmark is only using 10 nodes and completes in microseconds so there tends to be a bit of variance on it. This is actually what I would expect. Especially as our benchmarks currently only test layout computation, and don't benchmark tree creation at all. Whereas tree creation is the only thing I would expect this to affect (if anything).

Precisely. We should probably have dedicated benches for that.

Originally posted by @alice-i-cecile in #312 (comment)

Tree creation is useful to bench as it will impact latency when swapping between different screens (views) in end user apps.