Use a parallel sort to order txs while speculating

[ljedrz]

A small follow-up to https://github.com/AleoHQ/snarkVM/pull/2421 and the replacement of https://github.com/AleoHQ/snarkVM/pull/2424. 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

[howardwu]

Is the assumption that sort_unstable is safe because the IDs are supposed to be unique? If so, this should be documented in the code itself for future readers to know.

[ljedrz]

The unstable sort is safe, it only may reorder equal elements.

However, I realized that due to lookups, caching the key would be more meaningful here; I added a macro for sorting by cached key and adjusted this PR to use it.
I kept the macro for unstable sorting, as it may be useful in the future, for applications where they key extraction function is cheap.