runtime: chansend and chanrecv operations should not allocate

[niaow]

While working on #1142, I ran into the issue that channels may allocate. Since they are now becoming critical sections, this becomes more of an issue since this can cause the garbage collector to run with interrupts disabled.

We should modify the code generator to stack-allocate the channelBlockedList.

[aykevl]

#1144 has been merged, so this is fixed.