tools/fast: use -usecache

Author: medvednikov

cmd/tools/fast/fast.v

@@ -4,7 +4,7 @@
 import os
 import time
 
-const voptions = ' -skip-unused -show-timings -stats '
+const voptions = ' -skip-unused -usecache -show-timings -stats '
 
 fn main() {
 	exe := os.executable()
@@ -41,6 +41,7 @@ fn main() {
 	println('  Building vprod...')
 	exec('v -o $vdir/vprod -prod -prealloc $vdir/cmd/v')
 	// exec('v -o $vdir/vprod $vdir/cmd/v') // for faster debugging
+	exec('v -o v2 -prod -usecache $vdir/cmd/v') // cache vlib modules
 	diff1 := measure('$vdir/vprod $voptions -o v.c $vdir/cmd/v', 'v.c')
 	mut tcc_path := 'tcc'
 	$if freebsd {