Asynchronous WasmThemis initialization (#507)

It turns out that WebAssembly is compiled and loaded asynchronously [1].
There is an option for synchronous compilation, but Chrome and Electron
support only asynchronous loading. This means that one cannot simply
call C functions. We need to wait for WebAssembly to be compiled and
Emscripten runtime to boot. This is performed automatically if the whole
application is compiled by Emscripten, but Themis is a library.

Emscripten provides an "onRuntimeInitialized" callback which is called
when Emscripten startup is complete and C functions can be called.
Expose an interface to this callback as JavaScript promise which is
resolved once initialization is complete. The users should call Themis
functions only after this promise is resolved. (Alternatively, they can
wait for a second or two.)

This behavior is not reproduced by unit tests because Mocha launches
asynchronously too and it manages to start testing after WasmThemis
loading is complete. But we can test that the promise is resolved.

[1]: https://emscripten.org/docs/compiling/WebAssembly.html#wasm-files-and-compilation

src/wrappers/themis/wasm/README.md

@@ -35,6 +35,16 @@ Then import it into your project:
 const themis = require('wasm-themis')
 ```
 
+Finally, wait for WebAssembly code to be loaded:
+
+```javascript
+themis.initialized.then(function() {
+    //
+    // Now you can use "themis" functions
+    //
+})
+```
+
 ([_Browserify_ your code][browserify] to use WasmThemis in web apps.)
 
 [browserify]: http://browserify.org

src/wrappers/themis/wasm/src/index.js

@@ -17,6 +17,8 @@
  * WasmThemis module entry point.
  */
 
+const libthemis = require('./libthemis.js')
+
 Object.assign(module.exports
   , require('./secure_cell.js')
   , require('./secure_comparator.js')
@@ -25,3 +27,19 @@ Object.assign(module.exports
   , require('./secure_session.js')
   , require('./themis_error.js')
 )
+
+let resolveInitialization
+let initializationPromise = new Promise(function(resolve) {
+    resolveInitialization = resolve
+})
+
+/**
+ * Themis initialization promise.
+ *
+ * Resolved when Themis is loaded and ready to use.
+ */
+module.exports.initialized = initializationPromise
+
+libthemis["onRuntimeInitialized"] = function() {
+    resolveInitialization()
+}

src/wrappers/themis/wasm/test/test.js

@@ -33,6 +33,13 @@ describe('wasm-themis', function() {
         () => new Uint8Array([27, 18, 28, 18, 28]),
         { value: [3, 14, 15, 92, 6] }
     ]
+    describe('initialization', function() {
+        it('resolves "initialized" promise', function(done) {
+            themis.initialized.then(function() {
+                done()
+            })
+        })
+    })
     describe('KeyPair', function() {
         it('generates EC key pairs', function() {
             let pair = new themis.KeyPair()