time: microoptimise the Time formating methods (use custom number->string conversion, instead of string interpolation)

[enghitalo]

• no breaking changes, no new features

import time
import benchmark

const max_iterations = 100_000

fn main() {
	mut b := benchmark.start()

	for i := 0; i < max_iterations; i++ {
		time.utc().format_rfc3339()
	}

	b.measure("time.utc().format_rfc3339()")

	for i := 0; i < max_iterations; i++ {
		time.now().format_rfc3339() 
	}

	b.measure("time.now().format_rfc3339()")
}

Before

╰─ v -prod crun b.v ─╯
 SPENT    23.206 ms in time.utc().format_rfc3339()
 SPENT    42.505 ms in time.now().format_rfc3339()

After

╰─ v -prod crun b.v ─╯
 SPENT     6.150 ms in time.utc().format_rfc3339()
 SPENT    37.007 ms in time.now().format_rfc3339()

[spytheman]

Excellent work.