use left-grouping behavior of read-cdot

[AlexKnauth]

Goes with pull request https://github.com/racket/racket/pull/1446 on the Racket repository.

Instead of flattening (#%dot dt x ... (#%dot . y)) into (#%dot dt x ... . y), the new #%dot macro flattens (#%dot (#%dot dt x) . y) into (#%dot dt x . y).

[AlexKnauth]

The tests that passed before still pass and the tests that failed before still fail.

There are still errors, but they are the same errors that remix had with the right-grouping behavior before this pull request.

Before:

raco setup: error: during making for <pkgs>/remix/remix/tests
raco setup:   <pkgs>/remix/remix/tests/expand-from.rkt:4.0: def: expected def transformer
raco setup:     at: layout
raco setup:     in: (def (#%brackets layout posn) x y)
raco setup:     compiling: <pkgs>/remix/remix/tests/expand-from.rkt
raco setup: error: during making for <pkgs>/remix/remix/tests
raco setup:   <pkgs>/remix/remix/tests/expand-from.rkt:4.0: def: expected def transformer
raco setup:     at: layout
raco setup:     in: (def (#%brackets layout posn) x y)
raco setup: error: during making for <pkgs>/remix/remix/tests
raco setup:   /Users/Alex/Library/Racket/snapshot-6.6.0.4--2016-08-30/pkgs/remix/remix/tests/class.rkt:34:13: default-this: Unknown component '#:set, expected one of '()
raco setup:     at: #:set
raco setup:     in: (#%app (#%dot this #:set) (#%brackets c ((#%dot this (#%dot c #:set)) (#%brackets x (#%braces x + (#%dot this (#%dot c x)))) (#%brackets y (#%braces y + (#%dot this (#%dot c y)))))))
raco setup:     compiling: <pkgs>/remix/remix/tests/class.rkt

After:

raco setup: error: during making for <pkgs>/remix/remix/tests
raco setup:   <pkgs>/remix/remix/tests/expand-from.rkt:4.0: def: expected def transformer
raco setup:     at: layout
raco setup:     in: (def (#%brackets layout posn) x y)
raco setup:     compiling: <pkgs>/remix/remix/tests/expand-from.rkt
raco setup: error: during making for <pkgs>/remix/remix/tests
raco setup:   <pkgs>/remix/remix/tests/expand-from.rkt:4.0: def: expected def transformer
raco setup:     at: layout
raco setup:     in: (def (#%brackets layout posn) x y)
raco setup:     compiling: <pkgs>/remix/remix/tests/expand-from.rkt
raco setup: error: during making for <pkgs>/remix/remix/tests
raco setup:   /Users/Alex/Documents/GitHub/remix/remix/tests/class.rkt:34:13: default-this: Unknown component '#:set, expected one of '()
raco setup:     at: #:set
raco setup:     in: (#%app (#%dot this #:set) (#%brackets c ((#%dot (#%dot this c) #:set) (#%brackets x (#%braces x + (#%dot (#%dot this c) x))) (#%brackets y (#%braces y + (#%dot (#%dot this c) y))))))
raco setup:     compiling: <pkgs>/remix/remix/tests/class.rkt