Order script_purpose.compare by the ledger's script purpose ordering

[MavenRain]

Fixes #125.

Problem

cardano/transaction/script_purpose.compare follows the constructor order of ScriptPurpose (Mint first, Withdraw before Publish). The redeemers list in the script context follows the internal ledger's ordering of script purposes instead: spending purposes come first, and certificate publications come before withdrawals. So sorting or searching redeemers with compare disagrees with the on-chain order.

Fix

• Reorder the constructor arms of compare to the ledger ordering: Spend < Mint < Publish < Withdraw < Vote < Propose. The within-constructor comparators already match the ledger (inputs are a set ordered by output reference, mint policies are ordered lexicographically, Publish/Propose carry their pointer index, and credential.compare already puts Script below VerificationKey as reward accounts are ordered).
• Flip the four affected expectations in compare_matrix (done first, to confirm they fail against the old implementation).
• Reword the redeemers doc note on Transaction, and document the ordering on compare itself.
• CHANGELOG entry with the breaking-change marker.

Prepared against next since the issue is milestoned v4.0.0; I'd be happy to retarget main instead.

Testing

aiken check: 977 passed, 0 failed. With only the test flips applied, exactly those four assertions fail, so the matrix does exercise the new ordering.