Use stack allocation in TrSpendInfo

[lmorett1]

Replaces the heap-allocated Vec in nodes_from_tap_tree with a fixed-size stack array, addressing an existing FIXME.

Since Taproot tree depth is strictly consensus-limited to 128, using a small stack array (~2KB) eliminates unnecessary heap allocations during tree traversal without any risk of overflow.

[tcharding]

You can run just fmt to get past CI mate.

[tcharding]

ACK 94eea85

Code review only, I do not know about the 128 depth limit thing.

[tcharding]

ACK 90b1be3

[tcharding]

Is there any reason you didn't use ArrayVec like the code comment mentioned?

[apoelstra]

We don't have an arrayvec dependency here. (There is one which is inherited from hex-conservative 0.2 but it's not exposed and won't obviously be retained when we move to 1.0.)

This code looks fine to me but I'd prefer that it used TAPROOT_CONTROL_MAX_NODE_COUNT rather than 128, as in taptree.rs, so that the two bounds checks are comparable.