Substantially increase context_t stack size

It's nearly free to do so, because it's just virtual address space.

@davidbiancolin recently fell into this pit when using context_t with VCS.

fesvr/context.cc

@@ -49,7 +49,7 @@ void context_t::init(void (*f)(void*), void* a)
 #ifdef USE_UCONTEXT
   getcontext(context.get());
   context->uc_link = creator->context.get();
-  context->uc_stack.ss_size = 64*1024;
+  context->uc_stack.ss_size = 1024 * 1024;
   context->uc_stack.ss_sp = new void*[context->uc_stack.ss_size/sizeof(void*)];
 #ifndef GLIBC_64BIT_PTR_BUG
   makecontext(context.get(), (void(*)(void))&context_t::wrapper, 1, this);