Initial library implementations and tests (#1)

- [x] Implements library according to instructions, with tests.
- [x] GitHub Actions workflow to lint and test library.

Reviewed-on: #1

src/main/kotlin/tinyvm/Commit.kt

@@ -0,0 +1,34 @@
+package tinyvm
+
+import java.security.MessageDigest
+import java.time.Instant
+
+abstract class Object(
+    val type: String,
+) {
+    abstract val data: String
+
+    fun hash(): String =
+        MessageDigest
+            .getInstance("SHA-1")
+            .digest("$type ${data.length}\u0000$data".toByteArray())
+            .toHex()
+}
+
+class Commit(
+    val tree: Tree,
+    val author: Author,
+    val message: String,
+    val timestamp: Instant,
+) : Object("commit") {
+    // Use \n\n for end of header in-case additional metadata is implemented in the future.
+    override val data: String
+        get() = "tree ${tree.hash()}\nauthor $author\ntimestamp ${timestamp.epochSecond}\n\n$message"
+}
+
+data class Author(
+    val name: String,
+    val email: String,
+) {
+    override fun toString(): String = "$name <$email>"
+}