Add dice::tree and dice::TreeBuilder

[jakoschiko]

First attempt to provide a generator for tree-like structures.

[jakoschiko]

Here some stats for the max depth of generated trees with 100k nodes.

Branching:

dice::weighted_one_of_die().three(
    (16, dice::just(1)),
    (4, dice::just(2.min(max_branch_count))),
    (1, dice::uni_usize(1..=max_branch_count)),
)

Result:

The test withstood 10000 passes.

# Config
- seed: 13442140669564993173
- start limit: 0
- end limit: 100
- passes: 10000

# Stats
- max depth:
	- 15.51% (1551): ~110
	- 14.95% (1495): ~100
	- 11.71% (1171): ~120
	- 11.33% (1133): ~90
	- 9.39% (939): ~80
	- 7.92% (792): ~70
	- 6.83% (683): ~130
	- 6.79% (679): ~60
	- 4.61% (461): ~50
	- 3.40% (340): ~140
	- 2.81% (281): ~40
	- 1.44% (144): ~150
	- 1.34% (134): ~30
	- 0.75% (75): ~20
	- 0.50% (50): ~10
	- 0.35% (35): ~160
	- 0.21% (21): ~170
	- 0.13% (13): ~0
	- 0.02% (2): ~190
	- 0.01% (1): ~200