Preliminary feature staging

This partially implements the feature staging described in the
[release channel RFC][rc]. It does not yet fully conform to the RFC as
written, but does accomplish its goals sufficiently for the 1.0 alpha
release.

It has three primary user-visible effects:

* On the nightly channel, use of unstable APIs generates a warning.
* On the beta channel, use of unstable APIs generates a warning.
* On the beta channel, use of feature gates generates a warning.

Code that does not trigger these warnings is considered 'stable',
modulo pre-1.0 bugs.

Disabling the warnings for unstable APIs continues to be done in the
existing (i.e. old) style, via `#[allow(...)]`, not that specified in
the RFC. I deem this marginally acceptable since any code that must do
this is not using the stable dialect of Rust.

Use of feature gates is itself gated with the new 'unstable_features'
lint, on nightly set to 'allow', and on beta 'warn'.

The attribute scheme used here corresponds to an older version of the
RFC, with the `#[staged_api]` crate attribute toggling the staging
behavior of the stability attributes, but the user impact is only
in-tree so I'm not concerned about having to make design changes later
(and I may ultimately prefer the scheme here after all, with the
`#[staged_api]` crate attribute).

Since the Rust codebase itself makes use of unstable features the
compiler and build system to a midly elaborate dance to allow it to
bootstrap while disobeying these lints (which would otherwise be
errors because Rust builds with `-D warnings`).

This patch includes one significant hack that causes a
regression. Because the `format_args!` macro emits calls to unstable
APIs it would trigger the lint.  I added a hack to the lint to make it
not trigger, but this in turn causes arguments to `println!` not to be
checked for feature gates. I don't presently understand macro
expansion well enough to fix. This is bug #20661.

Closes #16678

[rc]: https://github.com/rust-lang/rfcs/blob/master/text/0507-release-channels.md

src/libsyntax/codemap.rs

@@ -590,7 +590,12 @@ impl CodeMap {
                 Some(ref info) => {
                     // save the parent expn_id for next loop iteration
                     expnid = info.call_site.expn_id;
-                    if info.callee.span.is_none() {
+                    if info.callee.name == "format_args" {
+                        // This is a hack because the format_args builtin calls unstable APIs.
+                        // I spent like 6 hours trying to solve this more generally but am stupid.
+                        is_internal = true;
+                        false
+                    } else if info.callee.span.is_none() {
                         // it's a compiler built-in, we *really* don't want to mess with it
                         // so we skip it, unless it was called by a regular macro, in which case
                         // we will handle the caller macro next turn