builtin,strconv: speedup str.int() conversions (without -prod)

vlib/v/tests/bench/bench_string_int.v

@@ -0,0 +1,17 @@
+import benchmark
+
+const maxn = 999_999
+
+fn main() {
+	mut snumbers := []string{cap: maxn}
+	for i in 0 .. maxn {
+		snumbers << i.str()
+	}
+	mut sum := i64(0)
+	mut bmark := benchmark.start()
+	for s in snumbers {
+		sum += s.int()
+	}
+	bmark.measure('s.int()')
+	dump(sum)
+}

vlib/v/tests/big_array_allocation_test.v

@@ -1,3 +1,5 @@
+import os
+
 [direct_array_access]
 fn test_big_int_array() {
 	dump(sizeof(isize))
@@ -5,6 +7,11 @@ fn test_big_int_array() {
 	if sizeof(isize) > 4 {
 		maxn = 1_000_000_000 // 1 billion integers, when each is 4 bytes => require ~4GB
 	}
+	// NB: this test requires RAM that many people do not have, so only run it in full, when VTEST_BIGMEM is 1
+	vtest_bigmem := os.getenv('VTEST_BIGMEM').int()
+	if vtest_bigmem == 0 {
+		maxn = 10_000_000
+	}
 	dump(maxn)
 	mut data := []int{len: maxn}