cgen: add support for -d trace_gen too. Add v tracev command.

cmd/v/help/default.txt

@@ -35,6 +35,9 @@ V supports the following commands:
 * Others:
    build             Build a V code in the provided path (the default, so you can skip the word `build`).
    translate         Translate C code to V (coming soon in 0.3).
+   tracev            Produce a tracing version of the v compiler.
+                     Use `tracev yourfile.v` when the compiler panics.
+                     NB: `tracev` is much slower and more verbose than ordinary `v`
 
 Use "v help <command>" for more information about a command, example: `v help build`, `v help build-c`
 Use "v help other" to see less frequently used commands.

cmd/v/v.v

@@ -12,7 +12,7 @@ import v.builder
 const (
 	simple_cmd = [
 		'fmt', 'up', 'vet',
-		'self', 'symlink', 'bin2v',
+		'self', 'tracev', 'symlink', 'bin2v',
 		'test', 'test-fmt', 'test-compiler', 'test-fixed',
 		'repl',
 		'build-tools', 'build-examples',