* Validate transparent coinbase output maturity and shielding
- Add a CoinbaseSpendRestriction enum and Transaction method
- Validate transparent coinbase spends in non-finalized chains
* Don't use genesis created UTXOs for spends in generated block chains
* Refactor out a new_transaction_ordered_outputs function
* Add Transaction::outputs_mut for tests
* Generate valid transparent spends in arbitrary block chains
* When generating blocks, fixup the block contents, then the block hash
* Test that generated chains contain at least one transparent spend
* Make generated chains long enough for reliable tests
* Add transparent and shielded input and output methods to Transaction
* Split chain generation into 3 functions
* Test that unshielded and immature transparent coinbase spends fail
* Comment punctuation
* Clarify a comment
* Clarify probability calculation
* Test that shielded mature coinbase output spends succeed
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