Bring Elements up to date with Bitcoin Core 0.21

[apoelstra]

Requires a bunch of followup work which I will open other issues for. See also #743 which was our checklist for 0.19.

Then in followup PRs:

• Fix the fuzz tests; I did not test these at any point throughout the rebase Post-0.21: fix fuzz tests #936
• Update and publish the merge-prs script to run the fuzzer, linter etc., as well as do a full clean make check WIP: Add merge-prs.sh script to contrib/ #923
• Audit translations; at a couple points I took Core's translations wholesale and we will need to re-edit them to change Bitcoin to Elements Post-0.21: audit translations #937
• Audit the GUI. I have not opened the bitcoin GUI since 0.12 and not opened Elements ever, so probably I broke this. Post-0.21: audit GUI #938
• Assess wallet/node coupling; Core did some separation in the build system which I undid, to support the signblock RPC which involves both wallet and node logic. Core's solution to this for Signet may be to introduce a separate signblock tool which we may want to also do; see #bitcoin-core-dev IRC logs for the meeting on 2020-12-03. Post-0.21: Separate wallet and node #939
• Assess functional tests: chase down a couple intermittent failures, assess the new broken loadblocks test, assess the "extended tests" which are broken on master Post-0.21: functional test failures #940
• Assess bitcoin functional tests and g_con_elementsmode Assess current value of g_con_elementsmode and consider removal #929
• Strengthen the functional tests; add more thorough PAK checks (see pak= format is undocumented and there is no functional test for it #926), move computefastmerkleroot into Python rather than using the RPC; Implement fast merkleroot computations in Python #941
• Fix the bumpfee RPC to support CT. There is an existing issue for this Fix bumpfee #527
• Fix the PAK standardness bug Pegouts are blocked by standardness rules if -enforce_pak is off on a network that uses PAK proofs #925 which was discovered through the rebase.
• Followup on conversion to bilingual_str from std::string for many Core RPCs; we should follow suit for our own RPCs Post-0.21: use bilingual_str rather than std::string in RPCs and elsewhere #942
• The new avoid_reuse param to sendtoaddress and a couple other RPCs moves some Elements fields out of the way. For backward compatibility we should put avoid_reuse after these Elements fields. (I think this is the only time I did this; after talking to Glenn I tried to make future RPC extensions backward compatible)
• Rebase Redesign PSET #900 on this and un-disable the large number of lines in the PSET functional tests which are disabled (in particular around the missing fee field of the RPC output)

Then a couple large followups. Tracking issues:

• Taproot: Taproot tracking issue #943
• Descriptors: Descriptors tracking issue #944
• Simplicity: Simplicity tracking issue #945