compiler,runtime: check for channel size limits

This patch is a combination of two related changes:

 1. The compiler now allows other types than `int` when specifying the
    size of a channel in a make(chan ..., size) call.
 2. The compiler now checks for maximum allowed channel sizes. Such
    checks are trivially optimized out in the vast majority of cases as
    channel sizes are usually constant.

I discovered this issue when trying out channels on AVR.

src/runtime/panic.go

@@ -50,6 +50,11 @@ func slicePanic() {
 	runtimePanic("slice out of range")
 }
 
+// Panic when trying to create a new channel that is too big.
+func chanMakePanic() {
+	runtimePanic("new channel is too big")
+}
+
 func blockingPanic() {
 	runtimePanic("trying to do blocking operation in exported function")
 }