Export Emscripten runtime for the Web (#509)

Some runtime environments look like Node.js, but are not Node.js which
confuses Emscripten loader code. I'm talking about JavaScript bundlers
that are used for Web and Electron apps. These things make the code
believe that it can use "require()" for importing and "module.exports"
for exporting things, and expect this be the case.

However, Emscripten believes that it is smarter, detects that it's not
running in Node.js environment, and just uses "Module" for exports.
This breaks bundler's expectation and various Emscripten runtime
functions (like "allocate()") end up not being exported, resulting in
weird TypeErrors when WasmThemis code tries calling them.

This can be avoided by exporting whatever Emscripten wants to export
in Node.js style via "module.exports". We can insert this code into
generated file by using "--pre-js" flag of emcc.

Since now we have more than one file for Emscripten compilation,
move them all into a subdirectory to keep the root directory tidy.

src/wrappers/themis/wasm/emscripten/pre.js

@@ -0,0 +1,4 @@
+// Manually export Emscripten runtime functions for weird environments that
+// look like Node.js, but aren't Node.js (e.g., Electron apps or output of
+// web bundlers like webpack or Browserify)
+module.exports = Module;

src/wrappers/themis/wasm/wasmthemis.mk

@@ -19,13 +19,16 @@ WASM_PATH = src/wrappers/themis/wasm
 WASM_SRC += $(WASM_PATH)/package.json
 WASM_SRC += $(wildcard $(WASM_PATH)/src/*.js)
 
-WASM_RUNTIME = $(abspath $(WASM_PATH)/runtime_exports.json)
+WASM_RUNTIME = $(abspath $(WASM_PATH)/emscripten/runtime_exports.json)
+WASM_PRE_JS  = $(abspath $(WASM_PATH)/emscripten/pre.js)
 
-$(BIN_PATH)/libthemis.js: LDFLAGS += -s EXTRA_EXPORTED_RUNTIME_METHODS=@$(WASM_RUNTIME) -s RESERVED_FUNCTION_POINTERS=1
+$(BIN_PATH)/libthemis.js: LDFLAGS += -s EXTRA_EXPORTED_RUNTIME_METHODS=@$(WASM_RUNTIME)
+$(BIN_PATH)/libthemis.js: LDFLAGS += -s RESERVED_FUNCTION_POINTERS=1
+$(BIN_PATH)/libthemis.js: LDFLAGS += --pre-js $(WASM_PRE_JS)
 
 $(BIN_PATH)/libthemis.js: CMD = $(CC) -o $@ $(filter %.o %a, $^) $(LDFLAGS)
 
-$(BIN_PATH)/libthemis.js: $(THEMIS_STATIC) $(WASM_RUNTIME)
+$(BIN_PATH)/libthemis.js: $(THEMIS_STATIC) $(WASM_RUNTIME) $(WASM_PRE_JS)
 	@mkdir -p $(@D)
 	@echo -n "link "
 	@$(BUILD_CMD)