time: optimize the performance of the function appendFormatRFC3339

[fx408]

Use AppendIntWidth4 instead of AppendInt to handle year Use AppendIntWidth2 instead of AppendInt to handle month, day, hour, minute, and second

Benchmark results: goos: windows goarch: amd64 pkg: time cpu: Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz

                 │   Before    │                After                │
                 │   sec/op    │   sec/op     vs base                │

TimeFormatRFC3339-12 40.91n ± 1% 25.47n ± 7% -37.74% (p=0.000 n=10)

            │  AppendInt  │           AppendIntWidth4           │
            │   sec/op    │   sec/op     vs base                │

AppendIntWidth4 4.973n ± 0% 3.541n ± 1% -28.80% (p=0.000 n=10)