Condition defining with operators other than equality

[Michał Buszkiewicz]

Prerequisite for #2.

We would like to be able to define queries like this:

def can(%{role: :visitor} = role) do
  grant(role)
  |> read(Record, approved: true, rating: {:>=, 5))
  |> read(Record, public: true)
end

Because we want Ecto query creation in #2, we would like to use operators that can be transformed to both plain-Elixir usage and transformation to Ecto queries:

• Comparison operators: ==, !=, <=, >=, <, > aliased to :eq, :neq, :gt, :gte, :lt, :lte
• Search functions: like/2 and ilike/2, mapped to appropriate regexps (expressions will have to be somehow translated from e.g. "%FOO" to /.*FOO/
• Regexp matching via SQL ~ (:~, :match) - for this purpose Ecto fragments will have to be used
• Null check functions: is_nil/1