Merge bitcoin/bitcoin#25877: refactor: Do not use CScript for tapleaf scripts until the tapleaf version is known

dee89438b82e94474ebaa31367035f98b4636dac Abstract out ComputeTapbranchHash (Russell O'Connor)
8e3fc9942729716e95907008fcf36eee758c3a6a Do not use CScript for tapleaf scripts until the tapleaf version is known (Russell O'Connor)

Pull request description:

  While BIP-341 calls the contents of tapleaf a "script", only in the case that the tapleaf version is `0xc0` is this script known to be a tapscript.  Otherwise the tapleaf "script" is simply an uninterpreted string of bytes.

  This PR corrects the issue where the type `CScript` is used prior to the tapleaf version being known to be a tapscript.  This prevents `CScript` methods from erroneously being called on non-tapscript data.

  A second commit abstracts out the TapBranch hash computation in the same manner that the TapLeaf computation is already abstracted.  These two abstractions ensure that the TapLeaf and TapBranch tagged hashes are always constructed properly.

ACKs for top commit:
  ajtowns:
    ACK dee89438b82e94474ebaa31367035f98b4636dac
  instagibbs:
    ACK dee89438b82e94474ebaa31367035f98b4636dac
  achow101:
    ACK dee89438b82e94474ebaa31367035f98b4636dac
  sipa:
    ACK dee89438b82e94474ebaa31367035f98b4636dac
  aureleoules:
    reACK dee89438b82e94474ebaa31367035f98b4636dac - I verified that there is no behavior change.

Tree-SHA512: 4a1d37f3e9a1890e7f5eadcf65562688cc451389581fe6e2da0feb2368708edacdd95392578d8afff05270d88fc61dce732d83d1063d84d12cf47b5f4633ec7e

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