feat(oid): add base bindings and api

lib/src/oid.dart

@@ -0,0 +1,58 @@
+import 'dart:ffi';
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/oid.dart' as bindings;
+import 'util.dart';
+
+class Oid {
+  /// Initializes a new instance of [Oid] class from provided
+  /// hexadecimal [sha] string.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Oid.fromSHA(String sha) {
+    libgit2.git_libgit2_init();
+
+    try {
+      _oidPointer = bindings.fromSHA(sha);
+    } catch (e) {
+      rethrow;
+    }
+  }
+
+  /// Pointer to memory address for allocated oid object.
+  late Pointer<git_oid> _oidPointer;
+
+  /// Returns hexadecimal SHA-1 string.
+  String get sha {
+    try {
+      return bindings.toSHA(_oidPointer);
+    } catch (e) {
+      rethrow;
+    }
+  }
+
+  @override
+  bool operator ==(other) {
+    return (other is Oid) &&
+        (bindings.compare(_oidPointer, other._oidPointer) == 0);
+  }
+
+  bool operator <(other) {
+    return (other is Oid) &&
+        (bindings.compare(_oidPointer, other._oidPointer) == -1);
+  }
+
+  bool operator <=(other) {
+    return (other is Oid) &&
+        (bindings.compare(_oidPointer, other._oidPointer) == -1);
+  }
+
+  bool operator >(other) {
+    return (other is Oid) &&
+        (bindings.compare(_oidPointer, other._oidPointer) == 1);
+  }
+
+  bool operator >=(other) {
+    return (other is Oid) &&
+        (bindings.compare(_oidPointer, other._oidPointer) == 1);
+  }
+}