Major changes

• Use cmd_die!() to replace die!() macro, which is more consistent for string interpolation within this library
• Add cmd_info!() macro to print messages conveniently
• Add basic redirection support for builtin/custom commands
• Update command registration API
• Change buf from String to Vec<u8> in CmdStdio struct
• Fix parsing escape characters
• Refactor lexer code to move iteration related information into a separate TokenStreamPeekable struct
• Support io input for builtin/custom commands, and add builtin cat as an example
• Support null command, like: run_cmd!(>/tmp/output)
• Print number literals without converting to actual value: run_cmd!(echo 0xff) will print "0xff" instead of "255".
• Make sure the children processes always being waited, even when the last command fails
• Don't panic process when there is any IO errors