improve Format() performance

[janishorsts]

This refactoring improves the performance of the Format().

$ go test -bench=. -benchmem -count=10 > a.txt # before changes
$ go test -bench=. -benchmem -count=10 > b.txt # after changes
$ benchstat a.txt b.txt                                 
goos: darwin
goarch: arm64
pkg: github.com/yaa110/go-persian-calendar
cpu: Apple M2 Max
          │    a.txt     │                b.txt                │
          │    sec/op    │   sec/op     vs base                │
Format-12   3440.0n ± 1%   822.5n ± 1%  -76.09% (p=0.000 n=10)

          │    a.txt    │               b.txt                │
          │    B/op     │    B/op     vs base                │
Format-12   5995.0 ± 0%   256.0 ± 3%  -95.73% (p=0.000 n=10)

          │   a.txt    │                b.txt                │
          │ allocs/op  │  allocs/op   vs base                │
Format-12   64.00 ± 2%   10.00 ± 10%  -84.38% (p=0.000 n=10)