Add asyncify bindings (AssemblyScript#972)

Author: dcodeIO

std/assembly/bindings/asyncify.ts

@@ -0,0 +1,16 @@
+@unmanaged
+export class StackDescriptor {
+  /** The index in linear memory of the start of the “asyncify stack”. */
+  stackStart: usize;
+  /** The index of the end of that stack region, which implies how big it is. */
+  stackEnd: usize;
+}
+
+/** Starts to unwind the call stack. */
+export declare function start_unwind(data: StackDescriptor): void;
+/** Stops unwinding the call stack. */
+export declare function stop_unwind(): void;
+/** Starts to rewind the call stack. */
+export declare function start_rewind(data: StackDescriptor): void;
+/** Stops rewinding the call stack. */
+export declare function stop_rewind(): void;