wire: remove FrameParser interface, expose FrameParser struct (#4284)

Instead, expose the FrameParser struct. This allows us to embed it
directly into the connection struct, avoiding a pointer indirection.

internal/wire/interface.go

@@ -9,9 +9,3 @@ type Frame interface {
 	Append(b []byte, version protocol.VersionNumber) ([]byte, error)
 	Length(version protocol.VersionNumber) protocol.ByteCount
 }
-
-// A FrameParser parses QUIC frames, one by one.
-type FrameParser interface {
-	ParseNext([]byte, protocol.EncryptionLevel, protocol.VersionNumber) (int, Frame, error)
-	SetAckDelayExponent(uint8)
-}