Provide methods for new slice helpers

[hadley]

And provide an informative error for the slice() method.

[hadley]

Look at #64 for implementation ideas.

[hadley]

• slice_head(), slice_tail()
• slice_min(), slice_max(),
• slice_sample()

All also work on groups, so I think will be translated into filter() calls.

# slice_min(x, n = 2)
lazy_frame(x = 1, g = 2) %>% 
  group_by(g) %>% 
  window_order(x) %>% 
  filter(min_rank() <= 2)

# slice_min(x, prop = 0.2)
lazy_frame(x = 1, g = 2) %>% 
  group_by(g) %>% 
  window_order(x) %>% 
  filter(cume_dist() <= 0.2)

# slice_min(x, n = 2, with_ties = FALSE)
lazy_frame(x = 1, g = 2) %>% 
  group_by(g) %>% 
  window_order(x) %>% 
  filter(row_number() <= 2)

# slice_min(x, prop = 0.2, with_ties = FALSE)
# error

# slice_max(x, n = 2)
lazy_frame(x = 1, g = 2) %>% 
  group_by(g) %>% 
  window_order(desc(x)) %>% 
  filter(min_rank() <= 2)

# slice_random(x, n = 2)
lazy_frame(x = 1, g = 2) %>% 
  group_by(g) %>% 
  window_order(RANDOM()) %>% 
  filter(min_rank() <= 2)