[breaking change] improve the select/3 API for key ranges

[lucaong]

The previous API had a few shortcomings:

• Corner cases for nil min_key/max_key and min_key/max_key literally being tuples like {123, :included}

• Misspelling :included or :excluded would silently fail or lead to unexpected results

• The API was a bit cumbersome in general

See also the discussion here: #3

The new API is much clearer, and removes the shortcomings:

# Exclusive ranges:
## Before:
{:ok, result} = CubDB.select(db,
  min_key: :foo,
  max_key: {:bar, :excluded}
)
## Now:
{:ok, result} = CubDB.select(db,
  min_key: :foo,
  max_key: :bar,
  max_key_inclusive: false
)

# Open-ended ranges:
## Before
{:ok, result} = CubDB.select(db,
  min_key: :foo,
  max_key: nil
)
## Now
{:ok, result} = CubDB.select(db,
  min_key: :foo
)

# Setting min/max key to `nil`:
## Before:
{:ok, result} = CubDB.select(db,
  min_key: {nil, :included},
  max_key: :bar
)
# Now:
{:ok, result} = CubDB.select(db,
  min_key: nil,
  max_key: :bar
)