vet the Makefile (#165)

- Move the emscripten target flags into the target configuration.
- Improve the readability of the Makefile.

Signed-off-by: xermicus <cyrill@parity.io>

.cargo/config.toml

@@ -0,0 +1,18 @@
+[target.wasm32-unknown-emscripten]
+rustflags = [
+    "-Clink-arg=-sEXPORTED_FUNCTIONS=_main,_free,_malloc",
+    "-Clink-arg=-sNO_INVOKE_RUN=1",
+    "-Clink-arg=-sEXIT_RUNTIME=1",
+    "-Clink-arg=-sALLOW_MEMORY_GROWTH=1",
+    "-Clink-arg=-sEXPORTED_RUNTIME_METHODS=FS,callMain,stringToNewUTF8",
+    "-Clink-arg=-sMODULARIZE=1",
+    "-Clink-arg=-sEXPORT_NAME=createRevive",
+    "-Clink-arg=-sWASM_ASYNC_COMPILATION=0",
+    "-Clink-arg=-sDYNAMIC_EXECUTION=0",
+    "-Clink-arg=-sALLOW_TABLE_GROWTH=1",
+    "-Clink-arg=--js-library=js/embed/soljson_interface.js",
+    "-Clink-arg=--pre-js=js/embed/pre.js",
+    "-Clink-arg=-sNODEJS_CATCH_EXIT=0",
+    "-Clink-arg=-sDISABLE_EXCEPTION_CATCHING=0",
+    "-Copt-level=3"
+]