Add contrib/justfile

Add a `justfile` containing useful development workflow commands.

Just recipes can be run by symlinking `contrib/justfile` into the
repository root:

    ln -s contrib/justfile justfile

And running:

    just RECIPE

Also add `/justfile` to `.gitignore`, to ignore symlinks.

`just` is command runner with make-like syntax. It is not a build
system, and only serves as convenient way of saving and running
commands. It is available here:

    https://github.com/casey/just/

Added in `contrib/justfile`, since `just` is a large, unreviewed
dependency. Developers considering using `just` should make sure to take
this into consideration.

.gitignore

@@ -21,3 +21,6 @@ test/lint/test_runner/target/
 /guix-build-*
 
 /ci/scratch/
+
+# ignore so users can symlink `contrib/justfile` into root
+/justfile

contrib/justfile

@@ -0,0 +1,28 @@
+# dependencies:
+#   just: https://github.com/casey/just
+#   watchexec: https://github.com/watchexec/watchexec
+
+set positional-arguments
+
+n := num_cpus()
+
+build: configure
+  cmake --build build -j {{ n }}
+
+configure:
+  @[[ -d build ]] || cmake -B build
+
+clean:
+  rm -rf build
+
+d *args:
+  ./build/src/bitcoind -regtest "$@"
+
+cli *args:
+  ./build/src/bitcoin-cli -regtest "$@"
+
+test: build
+  ctest --test-dir build -j {{ n }}
+
+watch:
+  watchexec 'just build'