Refactor - Transaction Input Types

[DaMatrix]

Description

This PR depends on #57 (tx-constructor-types), #44 (disable-p2sh), and #49 (addresses).

This replaces TxIn with an enum, where each variant contains the necessary information for a transaction input of a specific type. Each variant corresponds exactly to one of the original input script formats. Doing this brings the following advantages:

• Substantially more compact binary serialization, as each input contains only the minimal amount of information required to reconstruct the original unlock script instead of the full script
• Prevents accidental creation of a script with an invalid format from being created at all, which avoids accidental errors without breaking anything (because we reject every script whose format we don't recognize anyway)
• Enforces consistent type checks across the entire codebase, instead of re-declaring the types of each field in each kind of input script separately in every method
• Makes gross, hacky code which creates transaction inputs with empty scripts impossible
  • These have made their way into actual transaction objects in a number of tests
• Provides a clean mechanism to support alternative transaction formats in the future, including properly separating the locking and unlocking parts of a script between transaction inputs and outputs

Unfortunately, adding any kind of additional strict input validation when constructing these values is difficult/impossible, as it's impossible to check a single input validity's without knowing the context of the entire enclosing transaction and its input transactions. However, we can at least ensure that the individual fields have legal values thanks to strict typing, and we can ensure that we can't create weird inputs with impossible structures (such as a Coinbase input containing an OutPoint).

Changelog

• Added structs representing each type of transaction input, and an enum containing one variant for each
• Replaced the existing TxIn struct with the new enum
• Changed transaction_utils::construct_tx_in_out_signable_hash to accept an OutPoint instead of a TxIn
  • This function only ever used the OutPoint part of the TxIn, but because it needs to be called before a transaction input is created, there was a ton of code which had to create invalid TxIn instances with empty scripts

Type of Change

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• Bug fix
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• Documentation update
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• I have tested the changes locally and they work as expected.
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