feat(oid): add base bindings and api

lib/src/bindings/oid.dart

@@ -0,0 +1,44 @@
+import 'dart:ffi';
+import 'package:ffi/ffi.dart';
+import '../error.dart';
+import 'libgit2_bindings.dart';
+import '../util.dart';
+
+/// Parse a hex formatted object id into a git_oid.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_oid> fromSHA(String hex) {
+  final out = calloc<git_oid>();
+  final str = hex.toNativeUtf8().cast<Int8>();
+  final error = libgit2.git_oid_fromstr(out, str);
+  calloc.free(str);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out;
+  }
+}
+
+/// Format a git_oid into a hex string.
+///
+/// Throws a [LibGit2Error] if error occured.
+String toSHA(Pointer<git_oid> id) {
+  final out = calloc.allocate<Int8>(40);
+  final error = libgit2.git_oid_fmt(out, id);
+  final result = out.cast<Utf8>().toDartString(length: 40);
+  malloc.free(out);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return result;
+  }
+}
+
+/// Compare two oid structures.
+///
+/// Returns <0 if a < b, 0 if a == b, >0 if a > b.
+int compare(Pointer<git_oid> a, Pointer<git_oid> b) {
+  return libgit2.git_oid_cmp(a, b);
+}