whisper: remove linter warnings (#15972)

* whisper: fixes warnings from the code linter

* whisper: more non-API-breaking changes

The remaining lint errors are because of auto-generated
files and one is because an exported function has a non-
exported return type. Changing this would break the API,
and will be part of another commit for easier reversal.

* whisper: un-export NewSentMessage to please the linter

This is an API change, which is why it's in its own commit.
This change was initiated after the linter complained that
the returned type wasn't exported. I chose to un-export
the function instead of exporting the type, because that
type is an implementation detail that I would like to
change in the near future to make the code more
readable and with an increased coverage.

* whisper: update gencodec output after upgrading it to new lint standards

whisper/whisperv6/topic.go

@@ -23,11 +23,13 @@ import (
 	"github.com/ethereum/go-ethereum/common/hexutil"
 )
 
-// Topic represents a cryptographically secure, probabilistic partial
+// TopicType represents a cryptographically secure, probabilistic partial
 // classifications of a message, determined as the first (left) 4 bytes of the
 // SHA3 hash of some arbitrary data given by the original author of the message.
 type TopicType [TopicLength]byte
 
+// BytesToTopic converts from the byte array representation of a topic
+// into the TopicType type.
 func BytesToTopic(b []byte) (t TopicType) {
 	sz := TopicLength
 	if x := len(b); x < TopicLength {