rustc: Relax restriction on privacy for fields

This is a necessary change in preparation for switching the defaults as part
of #8122.

RFC: 0004-private-fields
syntax: Permit visibility on tuple fields

This change is in preparation for #8122. Nothing is currently done with these
visibility qualifiers, they are just parsed and accepted by the compiler.

RFC: 0004-private-fields
syntax: Tweak parsing bounds on generics paths

The previous syntax was `Foo:Bound<trait-parameters>`, but this is a little
ambiguous because it was being parsed as `Foo: (Bound<trait-parameters)` rather
than `Foo: (Bound) <trait-parameters>`

This commit changes the syntax to `Foo<trait-parameters>: Bound` in order to be
clear where the trait parameters are going.

Closes #9265
auto merge of #13145 : alexcrichton/rust/flip-some-defaults, r=brson

This change prepares `rustc` to accept private fields by default. These changes will have to go through a snapshot before the rest of the changes can happen.
auto merge of #13079 : alexcrichton/rust/colons, r=cmr

The previous syntax was `Foo:Bound<trait-parameters>`, but this is a little
ambiguous because it was being parsed as `Foo: (Bound<trait-parameters)` rather
than `Foo: (Bound) <trait-parameters>`

This commit changes the syntax to `Foo<trait-parameters>: Bound` in order to be
clear where the trait parameters are going.

Closes #9265
[std::cmp] add missing docs and provide an example