[ZIP-233] Network Sustainability Mechanism: Burning

[giddie]

This mostly consists of the addition of a burn_amount transaction field, introduced in the ZFuture network upgrade. Enabling cfg(zcash_unstable = "zfuture") meant that a lot of TZE code was unnecessarily enabled, so we switched some of those sections to zcash_unstable = "tze" as well as introducing zcash_unstable = "nsm" to keep things separate. The zfuture cfg is now used just to enable the ZFuture network upgrade.

The code for the generated test vectors is available in this PR.