Rebased by Pieter Wuille.
Cleanup by Matt Corallo.

Contains refactorings by Eric Lombrozo.
Contains fixup by Nicolas Dorier.

Includes simplifications by Eric Lombrozo.

Includes support for pushkeyhash wit v0 by Alex Morcos.

Includes fixup by Suhas Daftuar.

Includes fixup by Thomas Kerin.

…en verify_script receives VERIFY_WITNESS flag

script_tests: always test bitcoinconsensus_verify_script_with_amount if VERIFY_WITNESS isn't set

Rename internal method + make it static

trim bitcoinconsensus_ prefix

Add SERIALIZE_TRANSACTION_WITNESS flag

Witness blocks can be greater than 2MiB, but cannot be validly greater
than 4MB.

Break out witness commit position search and witness commitment and
nonce check into separate functions.

Rather than having the coinbase witness commitment be the hash of the
concatenation of the transaction witness merkle root and a single 32-byte
nonce from the coinbase witness, treat it instead as the merkle root
of a tree with the leaves being the transaction witness merkle root,
and each of the items from the coinbase stack.

For the case where the coinbase witness stack contains a single, 32-byte
item this change has no effect, however it should generalise well to allow
the coinbase witness stack to contain an arbitrary number of elements.

(If an element on the coinbase stack is not already exactly 32 bytes,
it is first hashed before being added as a leaf to the merkle tree)

This relaxes the restriction on the coinbase witness that limited it to
only having one entry that must be 32 bytes; instead there may be up to
255 items on the coinbase witness stack, and they may be any length.

In order to have multiple entries on the coinbase witness stack, the
number of entries must be specified in the coinbase commitment, by
changing the witness commitment from a 36 byte push:

  OP_RETURN [ 0xaa 0x21 0xa9 0xed HASH ]

to a 37 byte push:

  OP_RETURN [ 0xaa 0x21 0xa9 0xed HASH N ]

where N is the number of entries on the coinbase witness stack (from 0
to 255).