Drop unnecessary dependencies by moving interface type asserts to tests

[hsfzxjy]

Currently the following program has a rather large binary size of ~2.0M (on Linux):

package main

import (
	_ "github.com/Code-Hex/synchro"
)

func main() {
}

This is due to that code like below would introduce unnecessary dependencies to packages like encoding/gob:

var _ interface {
	fmt.Stringer
	fmt.GoStringer
	gob.GobEncoder
	gob.GobDecoder
	json.Marshaler
	json.Unmarshaler
	encoding.TextMarshaler
	encoding.TextUnmarshaler
	encoding.BinaryMarshaler
	encoding.BinaryUnmarshaler
} = (*synchro.Time[tz.UTC])(nil)

By moving all interface type asserts to tests, the binary size of the above program would drop to ~1.5M, which is considerable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fbe8d85) 100.00% compared to head (39cfbf9) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #32   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines         2635      2635           
=========================================
  Hits          2635      2635           

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[Code-Hex]

Thanks!