v,gen,checker: implement -d time_checking -d time_cgening -d time_v

cmd/v/help/build.txt

@@ -118,6 +118,12 @@ The build flags are shared by the build and run commands:
          CHECK: 62ms
          C GEN: 103ms
          C tcc: 95ms
+         
+      Related to -show-timings, is the ability to compile a special instrumented 
+      v compiler with this command: 
+      `v -d time_parsing -d time_checking -d time_cgening -d time_v self`
+      The instrumented version will print detailed timing stats while processing
+      each .v file.
 
    -w
       Hide all warnings.

cmd/v/v.v

@@ -37,6 +37,14 @@ const (
 )
 
 fn main() {
+	timers_should_print := $if time_v ? { true } $else { false }
+	mut timers := util.new_timers(timers_should_print)
+	timers.start('v total')
+	defer {
+		timers.show('v total')
+	}
+	timers.start('v start')
+	timers.show('v start')
 	args := os.args[1..]
 	// args = 123
 	if args.len == 0 || args[0] in ['-', 'repl'] {