Auto merge of #3180 - str4d:transaction-serialization, r=ebfull

Upstream serialization improvements

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#5264
- bitcoin/bitcoin#6914
- bitcoin/bitcoin#6215
- bitcoin/bitcoin#8068
  - Only the `COMPACTSIZE` wrapper commit
- bitcoin/bitcoin#8658
- bitcoin/bitcoin#8708
  - Only the serializer variadics commit
- bitcoin/bitcoin#9039
- bitcoin/bitcoin#9125
  - Only the first two commits (the last two block on other upstream PRs)

Part of #2074.

src/Makefile.am

@@ -167,6 +167,7 @@ BITCOIN_CORE_H = \
   paymentdisclosuredb.h \
   policy/fees.h \
   pow.h \
+  prevector.h \
   primitives/block.h \
   primitives/transaction.h \
   protocol.h \