get() and get_mut() by ref

See #7, release log:

• 72b332e by @yegor256: #7 contains_key by ref
• 605a354 by @yegor256: #7 clear()
• af29764 by @yegor256: #7 optim
• f749d65 by @yegor256: #7 get by ref
• 9dc39b4 by @yegor256: #6 debug and display for map

Released by Rultor 1.74.7, see build log