esp8266/main: Bump heap size to 36K.

ESP8266 SDK2.0 fixes (at least, I can't reproduce it) an infamous bug
with crash during scan. 36K seams to be a safe value based on a download
test (test_dl.py), over 1GB was downloaded. More testing is needed, but
let's have other people participate by committing it now.
diff --git a/esp8266/main.c b/esp8266/main.c
index 033ab7a..a2e747d 100644
--- a/esp8266/main.c
+++ b/esp8266/main.c
@@ -39,7 +39,7 @@
 #include "gccollect.h"
 #include "user_interface.h"
 
-STATIC char heap[28 * 1024];
+STATIC char heap[36 * 1024];
 
 STATIC void mp_reset(void) {
     mp_stack_set_top((void*)0x40000000);