Add reserved keyword proc

[ordovicia]

This PR adds proc to rustReservedKeyword.

[ordovicia]

Ping?

[pickfire]

What about yield?

[ordovicia]

yield is already added to rustKeyword here as well as macro and box.

I'm not sure they should be registered as rustReservedKeyword.
They are in "reserved keywords" list in the book but can be used in nightly (unless I'm misunderstanding).

[pickfire]

@ordovicia It's true that it's nightly-only but it's experimental right? My fault that I didn't check deeper since the docs showed that it belong to "Keywords Reserved for Future Use".

[ordovicia]

@pickfire Right. yield, macro, and box are all currently experimental (proc is not available even in nightly TMK).
@steveklabnik How do you think? Should these keywords belong to rustReservedKeyword?

[ordovicia]

The proc keyword is going to be removed from the reserved keywords list (rust-lang/rust#49699).

[ordovicia]

Closing this PR since the proc keyword has been removed.
IMHO other keywords e.g. yield should be discussed in a separate issue.