to_timedelta should parse strings like '1h' and '1 hour'

[shoyer]

Right now the only long name is supports is "days" and it doesn't even abbreviations for "m" or "h" for minute or hour.

All the logic to do the conversions already exists, it just needs better parsing.

Related: #7611

[jreback]

want to do a pr for this? (either before or after #8184 )