builtin: fix map.clear() not resetting map's metas and keys blocks (fix #22139) (#22140)

cmd/tools/bench/map_clear.v

@@ -0,0 +1,19 @@
+import benchmark
+
+fn main() {
+	max_iterations := arguments()[1] or { '1_000_000' }.int()
+	assert max_iterations > 0
+	mut m := {
+		123: 456
+		789: 321
+	}
+	mut volatile sum := u64(0)
+	mut b := benchmark.start()
+	for i in 0 .. max_iterations {
+		m.clear()
+		m[i] = i * 2
+		sum += u64(m.len)
+	}
+	assert m.len == 1
+	b.measure('m.clear(), iterations: ${max_iterations}, sum: ${sum}')
+}

cmd/tools/bench/map_clear_runner.vsh

@@ -0,0 +1,23 @@
+#!/usr/bin/env -S v -raw-vsh-tmp-prefix tmp
+
+import os
+
+const time_fmt = '"CPU: %Us\tReal: %es\tElapsed: %E\tRAM: %MKB\t%C"'
+const flags = os.getenv('FLAGS')
+
+unbuffer_stdout()
+
+start := os.args[1] or { '1_000_000' }.int()
+end := os.args[2] or { '10_000_000' }.int()
+step := os.args[3] or { '500_000' }.int()
+
+os.chdir(os.dir(@VEXE))!
+vcmd := 'v ${flags} cmd/tools/bench/map_clear.v'
+
+println('>> start: ${start} | end: ${end} | step: ${step} | workdir: "${os.getwd()}" | flags: "${flags}" | vcmd: "${vcmd}"')
+assert os.system(vcmd) == 0
+
+println('running...')
+for i := start; i <= end; i += step {
+	os.system('/usr/bin/time -f ${time_fmt} cmd/tools/bench/map_clear ${i}') == 0
+}