iter, runtime: add coroutine support

The exported API is only available with GOEXPERIMENT=rangefunc.
This will let Go 1.22 users who want to experiment with rangefuncs
access an efficient implementation of iter.Pull and iter.Pull2.

For #61897.

Change-Id: I6ef5fa8f117567efe4029b7b8b0f4d9b85697fb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/543319
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/runtime2.go

@@ -483,6 +483,7 @@ type g struct {
 	// inMarkAssist indicates whether the goroutine is in mark assist.
 	// Used by the execution tracer.
 	inMarkAssist bool
+	coroexit     bool // argument to coroswitch_m
 
 	raceignore    int8  // ignore race detection events
 	nocgocallback bool  // whether disable callback from C
@@ -507,6 +508,8 @@ type g struct {
 	timer         *timer         // cached timer for time.Sleep
 	selectDone    atomic.Uint32  // are we participating in a select and did someone win the race?
 
+	coroarg *coro // argument during coroutine transfers
+
 	// goroutineProfiled indicates the status of this goroutine's stack for the
 	// current in-progress goroutine profile
 	goroutineProfiled goroutineProfileStateHolder
@@ -1124,6 +1127,7 @@ const (
 	waitReasonFlushProcCaches                         // "flushing proc caches"
 	waitReasonTraceGoroutineStatus                    // "trace goroutine status"
 	waitReasonTraceProcStatus                         // "trace proc status"
+	waitReasonCoroutine                               // "coroutine"
 )
 
 var waitReasonStrings = [...]string{
@@ -1162,6 +1166,7 @@ var waitReasonStrings = [...]string{
 	waitReasonFlushProcCaches:       "flushing proc caches",
 	waitReasonTraceGoroutineStatus:  "trace goroutine status",
 	waitReasonTraceProcStatus:       "trace proc status",
+	waitReasonCoroutine:             "coroutine",
 }
 
 func (w waitReason) String() string {