distro-build/installer-dmg: codesign whole ".app" instead of just exe

This change is needed for the more stringent Gatekeeper introduced in
Mac OS X 10.9.5, but the build machine has to be 10.9 or later to make
10.9.5 clients happy.
Fix typo (does -> doesn't).
v6.1.1 announcement
v6.1.1 installers info
remove info for natipkg installers
fix the "Racket Documentation.app" icon
update "news" on main web page
Adjust contract generation for provides in TR

Use the same mechanism as ordinary require/typed to generate
contracts for provide forms.
Eliminate special cases for top-level contract gen

Use the same mechanism as require/typed for doing all
contract gen for make-predicate, cast, etc. Also don't
special-case contract generation for top-level require/typed.
Fix type of expt.

Add expt to random testing.

Haven't seen one of those bugs in a while. Thank you Redex.

Closes PR14823.
Fix other bugs in type of expt.

Found using the new random testing.
Change contract on change-contract-fixups

The context should provide a flattened list of syntax
to the function.
Avoid direct use of type->contract in with-type

Eliminates the last remaining use of type->contract
outside of the type-contract.rkt file. This allows
all contract generation to go through a single point
of control.
Simplify contract generation code

Don't export type->contract except for testing. This
discourages direct use of type->contract in favor of
using the change-contract-fixup approach.

Also consolidate most of the contract-related syntax
properties into a single property containing a prefab
struct instance.