Use a parallel sort to order txs while speculating

[ljedrz]

A small follow-up to https://github.com/AleoHQ/snarkVM/pull/2421. The changes are:

• a cfg_sort_unstable_by macro for parallel sorting using a closure is introduced (note: the unstable part only means that equal elements may be reordered, and transaction IDs are never equal)
• the aforementioned macro is used to sort transactions when speculating
• the parallel iteration for the collection is removed, as it is slower than single-threaded iteration (since Transaction::id is basically free)
• instead of enumerating and collecting indices, IndexSet::get_index_of is used

[ljedrz]

The CI jobs are failing due to a GitHub incident; will rerun a bit later.

[ljedrz]

It seems that for whatever reason the CI can't be triggered successfully after the GitHub failure; I'm going to open a new PR with a new branch name.