all: rename IsInitialized as CheckInitialized

An Is prefix implies it returns a boolean.
A Check prefix better suggests that it could return an error.

Change-Id: I6ffcb32099a944c656c07654c294a0980efb2d0e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220338
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/codec_map.go b/internal/impl/codec_map.go
index dfeb944..dd5f8c7 100644
--- a/internal/impl/codec_map.go
+++ b/internal/impl/codec_map.go
@@ -320,7 +320,7 @@
 		iter := mapRange(mapv)
 		for iter.Next() {
 			val := pointerOfValue(iter.Value())
-			if err := mi.isInitializedPointer(val); err != nil {
+			if err := mi.checkInitializedPointer(val); err != nil {
 				return err
 			}
 		}