feat: speed up ProtoMethods

The ProtoMethods is called for every unmarshal, which in turn allocates
an protoiface.Methods. The Methods object does not refer to a partucular
message object, so the allocation can be saved by one-time initialization
at startup.

Found by the the alecthomas/go_serialization_benchmarks suite:

    go test -bench='Pulsar' -memprofile mem.out  -cpu 6  ./

Before:
goos: darwin
goarch: arm64
pkg: github.com/alecthomas/go_serialization_benchmarks
Benchmark_Pulsar_Marshal-6     	 7480744	       154.1 ns/op	        51.57 B/serial	      96 B/op	       2 allocs/op
Benchmark_Pulsar_Unmarshal-6   	 5057922	       236.9 ns/op	        51.60 B/serial	     256 B/op	       7 allocs/op

After:
Benchmark_Pulsar_Marshal-6     	 7251019	       153.9 ns/op	        51.57 B/serial	      96 B/op	       2 allocs/op
Benchmark_Pulsar_Unmarshal-6   	 6409070	       187.2 ns/op	        51.63 B/serial	     160 B/op	       5 allocs/op

features/fastreflection/proto_message.go

@@ -288,19 +288,26 @@ func (g *fastGenerator) genIsValid() {
 }
 
 func (g *fastGenerator) genProtoMethods() {
+	varName := g.typeName + "ProtoMethods"
+
 	g.P("// ProtoMethods returns optional fastReflectionFeature-path implementations of various operations.")
 	g.P("// This method may return nil.")
 	g.P("//")
 	g.P("// The returned methods type is identical to")
 	g.P(`// "google.golang.org/protobuf/runtime/protoiface".Methods.`)
 	g.P("// Consult the protoiface package documentation for details.")
-	g.P("func (x *", g.typeName, ") ProtoMethods() *", protoifacePkg.Ident("Methods"), " {")
+	g.P("func (*", g.typeName, ") ProtoMethods() *", protoifacePkg.Ident("Methods"), " {")
+	g.P("return ", varName)
+	g.P("}")
+
+	g.P("var ", varName, " *", protoifacePkg.Ident("Methods"))
 
+	g.P("func init() {")
 	g.genSizeMethod()
 	g.genMarshalMethod()
 	g.genUnmarshalMethod()
 
-	g.P("return &", protoifacePkg.Ident("Methods"), "{ ")
+	g.P(varName, " = &", protoifacePkg.Ident("Methods"), "{ ")
 	g.P("NoUnkeyedLiterals: struct{}{},")
 	g.P("Flags: ", protoifacePkg.Ident("SupportMarshalDeterministic"), "|", protoifacePkg.Ident("SupportUnmarshalDiscardUnknown"), ",")
 	g.P("Size: size,")