btcutil: move btcutil into new sub-module

In this commit, we move `btcutil` as well as its sub-module, the `psbt`
package into the `btcd` repo itself.

btcutil/LICENSE

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+ISC License
+
+Copyright (c) 2013-2017 The btcsuite developers
+Copyright (c) 2016-2017 The Lightning Network Developers
+
+Permission to use, copy, modify, and distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

btcutil/README.md

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+btcutil
+=======
+
+[![Build Status](https://github.com/btcsuite/btcd/btcutil/workflows/Build%20and%20Test/badge.svg)](https://github.com/btcsuite/btcd/btcutil/actions)
+[![ISC License](https://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org)
+[![GoDoc](https://img.shields.io/badge/godoc-reference-blue.svg)](https://godoc.org/github.com/btcsuite/btcd/btcutil)
+
+Package btcutil provides bitcoin-specific convenience functions and types.
+A comprehensive suite of tests is provided to ensure proper functionality.  See
+`test_coverage.txt` for the gocov coverage report.  Alternatively, if you are
+running a POSIX OS, you can run the `cov_report.sh` script for a real-time
+report.
+
+This package was developed for btcd, an alternative full-node implementation of
+bitcoin which is under active development by Conformal.  Although it was
+primarily written for btcd, this package has intentionally been designed so it
+can be used as a standalone package for any projects needing the functionality
+provided.
+
+## Installation and Updating
+
+```bash
+$ go get -u github.com/btcsuite/btcd/btcutil
+```
+
+## GPG Verification Key
+
+All official release tags are signed by Conformal so users can ensure the code
+has not been tampered with and is coming from the btcsuite developers.  To
+verify the signature perform the following:
+
+- Download the public key from the Conformal website at
+  https://opensource.conformal.com/GIT-GPG-KEY-conformal.txt
+
+- Import the public key into your GPG keyring:
+  ```bash
+  gpg --import GIT-GPG-KEY-conformal.txt
+  ```
+
+- Verify the release tag with the following command where `TAG_NAME` is a
+  placeholder for the specific tag:
+  ```bash
+  git tag -v TAG_NAME
+  ```
+
+## License
+
+Package btcutil is licensed under the [copyfree](http://copyfree.org) ISC
+License.