9d2005285c doc: Revise comments and whitespace to clarify (Ben Woosley)
def43a4d88 refactor: Rename i to curr_try in SelectCoinsBnB (Ben Woosley)
1dd0923677 refactor: Track BnB selection by index (Ben Woosley)
Pull request description:
This is prompted by #13167 and presented as a friendly alternative to it.
IMO you can improve code readability and performance by about 20% by tracking the selected utxos by index, rather than by position. This reduces the storage access complexity from roughly O(utxo_size) to O(selection_size).
On my machine (median of 5 trials):
```
BnBExhaustion, 5, 650, 2.2564, 0.000672999, 0.000711565, 0.000693112 - master
BnBExhaustion, 5, 650, 1.76232, 0.000528563, 0.000568806, 0.000539147 - this PR
```
ACKs for top commit:
achow101:
reACK 9d2005285c
glozow:
code review ACK 9d2005285c
Xekyo:
reACK 9d2005285c
Tree-SHA512: 453ea11ad58c48928dc76956e3e98916f6924e95510eb02fe89a899ff102fe9cc08a04d557f381ad0218a210275e5383101d971c1ffad38b06b1c57d81144315
e66630cc87