Add a justfile

The `just` command makes scripts and commands discoverable for new devs
and old devs alike when switching between repos.

Add a justfile copied from bitcoin with changes as required.

justfile

@@ -0,0 +1,30 @@
+default:
+  @just --list
+
+# Cargo build everything.
+build:
+  cargo build --all-targets --all-features
+
+# Cargo check everything.
+check:
+  cargo check --all-targets --all-features
+
+# Lint everything.
+lint:
+  cargo clippy --all-targets --all-features -- --deny warnings
+
+# Check the formatting
+format:
+  cargo +nightly fmt --check
+
+# Quick and dirty CI useful for pre-push checks.
+sane: lint
+  cargo test --quiet --all-targets --no-default-features > /dev/null || exit 1
+  cargo test --quiet --all-targets > /dev/null || exit 1
+  cargo test --quiet --all-targets --all-features > /dev/null || exit 1
+
+  # doctests don't get run from workspace root with `cargo test`.
+  cargo test --quiet --doc || exit 1
+
+  # Make an attempt to catch feature gate problems in doctests
+  cargo test --manifest-path Cargo.toml --doc --no-default-features > /dev/null || exit 1