feat(store): add a mock context constructor needed for tests

Used in the fuzzer implementation: #133

src/grammar/store.ts

@@ -42,4 +42,19 @@ export function openContext(ctx: CompilerContext,
     }
     ctx = store.set(ctx, 'types', { sources, funcSources, functions, constants, types });
     return ctx;
-}
+}
+
+// Creates a mock context with the given AST elements needed for testing
+// purposes
+export function openMockContext(
+    ctx: CompilerContext,
+    types: ASTType[],
+    functions: (ASTNativeFunction|ASTFunction)[],
+    constants: ASTConstant[],
+) {
+  const sources: {code: string, path: string}[] = [];
+  const funcSources: {code: string, path: string}[] = [];
+  ctx = store.set(ctx, 'types',
+                  {sources, funcSources, functions, constants, types});
+  return ctx;
+}