Eliminate intermediate allocs when using StdTime/StdDuration

[timpalpant]

When using StdTime/StdDuration, we incur unnecessary allocations due to the use of an intermediate *Timestamp/*Duration that escapes to the heap from a potential call to fmt.Errorf. This can be seen in:

$ go build -a -gcflags='-m -m' timestamp*

./timestamp_gogo.go:84:8: &Timestamp literal escapes to heap
./timestamp_gogo.go:84:8: 	from ts (assigned) at ./timestamp_gogo.go:84:5
./timestamp_gogo.go:84:8: 	from ts (passed to call[argument escapes]) at ./timestamp_gogo.go:88:31

./timestamp.go:66:21: ts escapes to heap
./timestamp.go:66:21: 	from ... argument (arg to ...) at ./timestamp.go:66:20
./timestamp.go:66:21: 	from *(... argument) (indirection) at ./timestamp.go:66:20
./timestamp.go:66:21: 	from ... argument (passed to call[argument content escapes]) at ./timestamp.go:66:20
./timestamp.go:61:24: leaking param: ts
./timestamp.go:61:24: 	from ts (interface-converted) at ./timestamp.go:66:21
./timestamp.go:61:24: 	from ... argument (arg to ...) at ./timestamp.go:66:20
./timestamp.go:61:24: 	from *(... argument) (indirection) at ./timestamp.go:66:20
./timestamp.go:61:24: 	from ... argument (passed to call[argument content escapes]) at ./timestamp.go:66:20

$ go build -a -gcflags='-m -m' duration*

./duration_gogo.go:90:9: &Duration literal escapes to heap
./duration_gogo.go:90:9: 	from dur (assigned) at ./duration_gogo.go:90:6
./duration_gogo.go:90:9: 	from dur (passed to call[argument escapes]) at ./duration_gogo.go:94:30

./duration.go:59:21: d escapes to heap
./duration.go:59:21: 	from ... argument (arg to ...) at ./duration.go:59:20
./duration.go:59:21: 	from *(... argument) (indirection) at ./duration.go:59:20
./duration.go:59:21: 	from ... argument (passed to call[argument content escapes]) at ./duration.go:59:20
./duration.go:54:23: leaking param: d
./duration.go:54:23: 	from d (interface-converted) at ./duration.go:59:21
./duration.go:54:23: 	from ... argument (arg to ...) at ./duration.go:59:20
./duration.go:54:23: 	from *(... argument) (indirection) at ./duration.go:59:20
./duration.go:54:23: 	from ... argument (passed to call[argument content escapes]) at ./duration.go:59:20

This is a major source of GC pressure for me (see also #254). This change eliminates the potential escape by introducing two small functions formatTimestamp and formatDuration to perform the equivalent formatting (for these specific cases) instead of using fmt.Errorf.

There are tests added to verify the equivalence, as well as to verify zero allocs in SizeOf, MarshalTo, and Unmarshal for both StdTime and StdDuration.

[jacksontj]

Having looked at more of the code, do we want to even keep this as a *Timestamp as opposed to just a Timestamp? If the timestamp is nil its an error, so that sounds like the pointer is not needed (which also leads to some of these methods actually moving it to heap.