tools: implement an -s option for v ast, to skip all nodes with default values like [], {}, 0, false; with it v ast -pts examples/hello_world.v | wc -l is 36 lines

[spytheman]

The output with the new -s option is a lot more terse, which can help with early diagnostics of parser problems.

#0 15:47:39 ^ master ~/code/v>./v ast -p examples/hello_world.v |wc
    180     335    3282
#0 15:48:28 ^ master ~/code/v>./v ast -pt examples/hello_world.v |wc
     75     141    1555
#0 15:48:29 ^ master ~/code/v>./v ast -pts examples/hello_world.v |wc
     36      64     643
#0 15:48:31 ^ master ~/code/v>

Output for a simple program:

#0 15:55:34 ^ master ~/code/v>v ast -pts examples/hello_world.v | wc -l
36
#0 15:55:35 ^ master ~/code/v>v ast -pts examples/hello_world.v
{
	"ast_type":	"ast.File",
	"path":	"/home/delian/code/v/examples/hello_world.v",
	"nr_lines":	1,
	"nr_bytes":	25,
	"mod":	{
		"ast_type":	"Module",
		"name":	"main",
		"is_skipped":	true
	},
	"stmts":	[{
			"ast_type":	"Module",
			"name":	"main",
			"is_skipped":	true
		}, {
			"ast_type":	"ExprStmt",
			"expr":	{
				"ast_type":	"CallExpr",
				"mod":	"main",
				"name":	"println",
				"left":	{
					"ast_type":	"NodeError"
				},
				"args":	[{
						"ast_type":	"CallArg",
						"expr":	{
							"ast_type":	"StringLiteral",
							"val":	"Hello, World!"
						}
					}],
				"or_block":	{
					"ast_type":	"OrExpr"
				}
			}
		}]
}
#0 15:55:38 ^ master ~/code/v>

[huly-for-github]

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