Make union/except/intersect chainable

[fatkodima]

Fixes #118.

This is an updated and changed/simplified version of #320.
The idea remains the same (SelectManager returns a new SelectManager on calls to union/intersect/except with, basically speaking, N-ary Union/Intersect/Excepts taking place of SelectCores inside SelectStatement).

TOP related code for SelectStatement was removed because it is basically a noop operation and will require implement also noop method for Union/.../... classes to work here

arel/lib/arel/select_manager.rb

Line 224 in df2b74b

@ctx.top = Nodes::Top.new(limit)

Also, squashing braces in unions for MySQL was removed to allow applying limits and orders for individual queries. As stated in documentation:

To apply ORDER BY or LIMIT to an individual SELECT, place the clause inside the parentheses that enclose the SELECT.

Note: Previous versions of MySQL may permit such statements without parentheses. In MySQL 5.7, the requirement for parentheses is enforced.

But was introduced for SQLite, as it has troubles with parenthesized union subqueries and ordering + limits in them.

I'm also wondering, should all changed behavior be extensively deprecated or this PR can be merged in the next major version without any?

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[matthewd]

Per #523, Arel development is moving to rails/rails.

If this PR is still relevant, please consider reopening it over there.