Use our new syntax extension loading machinery to remove the hardcoded macros in libsyntax

[bstrie]

< bstrie_remorse> now that we have loadable syntax extensions, are we actually
                  using them for e.g. println! or are we still hardcoding those
                  into libsyntax?
<@acrichto> not yet
<@acrichto> not yet using them
<@acrichto> I don't think there's a blocker for using them though

We've got a lot of syntax extensions hardcoded into src/libsyntax/ext. That shouldn't be necessary anymore. Fix this terrible hack!

[sfackler]

Pulling the procedural macros out (format, bytes, etc) will require some thought to get it to bootstrap properly. The macro_rules macros should be easily extractible. #11720 is a blocker imo, but it should be merging in today.

[steveklabnik]

While this situation isn't perfect, @sfackler has moved the macros out of libsyntax and into libstd.

[steveklabnik]

Triage: I am not aware of any more changes on this front.

[bstrie]

I'm content to close this now, what's left is just cleanup work.