All kinds of things

- Add modified BSD license
- Initial test runner github action
- Add is_{type} functions
- export kiwi.Error metatable such that it appears like a class.
- Allow Expression.new to take nil for terms
- Initial set of specs

.github/workflows/busted.yml

@@ -0,0 +1,36 @@
+name: Busted
+
+on: [push, pull_request]
+
+jobs:
+  busted:
+    strategy:
+      fail-fast: false
+      matrix:
+        lua_version: ["luajit-2.1.0-beta3", "luajit-2.0.5", "luajit-openresty"]
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup ‘lua’
+        uses: leafo/gh-actions-lua@v10
+        with:
+          luaVersion: ${{ matrix.lua_version }}
+      - name: Setup ‘luarocks’
+        uses: leafo/gh-actions-luarocks@v4
+      - name: Setup dependencies
+        run: |
+          luarocks install busted
+          luarocks install luacov-coveralls
+      - name: Build C library
+        run: make
+      - name: Run busted tests
+        run: busted -c -v
+      - name: Report test coverage
+        if: success()
+        continue-on-error: true
+        run: luacov-coveralls -e .luarocks -e spec
+        env:
+          COVERALLS_REPO_TOKEN: ${{ github.token }}