Increase asyncify stack size (#106)

Pick the first power of two where we fail with a pystack exhausted
error in MicroPython rather than an asyncify-related error.

Add an example to the demo that finds the max stack size.

src/Makefile

@@ -44,6 +44,8 @@ COPT += -O3 -DNDEBUG
 endif
 
 JSFLAGS += -s ASYNCIFY
+# We can hit lower values due to user stack use. See stack_size.py example.
+JSFLAGS += -s ASYNCIFY_STACK_SIZE=262144
 JSFLAGS += -s EXIT_RUNTIME
 JSFLAGS += -s MODULARIZE=1
 JSFLAGS += -s EXPORT_NAME=createModule

src/demo.html

@@ -100,6 +100,7 @@ <h1>MicroPython-micro:bit simulator example embedding</h1>
               </option>
               <option value="sound_effects_user">Sound effects (user)</option>
               <option value="speech">Speech</option>
+              <option value="stack_size">Stack size</option>
               <option value="volume">Volume</option>
             </select>
             <textarea

src/examples/stack_size.py

@@ -0,0 +1,10 @@
+def g():
+    global depth
+    depth += 1
+    g()
+depth = 0
+try:
+    g()
+except RuntimeError:
+    pass
+print('maximum recursion depth g():', depth)