fix build

* `EXPORT_ES6` and `ENVIRONMENT=*node*` requires `USE_ES6_IMPORT_META` to be set.

* `_malloc` and `_free` must be declared as exported functions to avoid removal by the compiler.

* Remove [`WASM_ASYNC_COMPILATION=0`](https://emsettings.surma.technology/#WASM_ASYNC_COMPILATION).

  The current code was broken by emscripten-core/emscripten#12650. Previously, `WASM_ASYNC_COMPILATION=0` would return `Promise<Module>`, but that PR made it return `Module` directly. (Took ages to find the cause of this!)

  Removing the flag to keep it async avoids a breaking change in this library. Node.js v14.8.0 and later support top-level await, so async isn't that ugly. Alternatively, the Node.js version could easily be changed to load synchronously. Node.js has no limit on the size of synchronously loaded WebAssembly modules, unlike Chromium.

Makefile

@@ -7,9 +7,8 @@ EMX_FLAGS :=
 EMX_FLAGS += -Os
 EMX_FLAGS += -s EXPORT_ES6=1
 EMX_FLAGS += -s MODULARIZE=1
-EMX_FLAGS += -s USE_ES6_IMPORT_META=0
+EMX_FLAGS += -s USE_ES6_IMPORT_META=1
 EMX_FLAGS += -s EXPORT_NAME='promise'
-EMX_FLAGS += -s WASM_ASYNC_COMPILATION=0
 EMX_FLAGS += -s ALLOW_MEMORY_GROWTH=1
 EMX_FLAGS += -s ALLOW_TABLE_GROWTH=1
 EMX_FLAGS += -s INITIAL_MEMORY=8MB
@@ -31,14 +30,14 @@ node: lmfit
 	emcc $(EMX_FLAGS) -I$(LMFIT_SRC)/lib $(LMFIT_SRC)/build/lib/liblmfit.a src/lmfit.js.c \
 	-s ENVIRONMENT="node" \
 	-s EXPORTED_RUNTIME_METHODS="[addFunction, removeFunction, getValue, cwrap]" \
-	-s EXPORTED_FUNCTIONS="[_do_fit]" \
+	-s EXPORTED_FUNCTIONS="[_do_fit, _malloc, _free]" \
 	-o $(BUILD_DIR)/lmfit.js;
 
 web: lmfit
 	emcc $(EMX_FLAGS) -I$(LMFIT_SRC)/lib $(LMFIT_SRC)/build/lib/liblmfit.a src/lmfit.js.c \
 	-s ENVIRONMENT="worker" \
 	-s EXPORTED_RUNTIME_METHODS="[addFunction, removeFunction, getValue, cwrap]" \
-	-s EXPORTED_FUNCTIONS="[_do_fit]" \
+	-s EXPORTED_FUNCTIONS="[_do_fit, _malloc, _free]" \
 	-o $(BUILD_DIR)/lmfit.web.js;
 
 lmfit: dir lmfit-patch