Hack for "unsafety hygiene" -- push_unsafe! and pop_unsafe!.

Even after expansion, the generated expressions still track depth of
such pushes (i.e. how often you have "pushed" without a corresponding
"pop"), and we add a rule that in a context with a positive
`push_unsafe!` depth, it is effectively an `unsafe` block context.

(This way, we can inject code that uses `unsafe` features, but still
contains within it a sub-expression that should inherit the outer
safety checking setting, outside of the injected code.)

This is a total hack; it not only needs a feature-gate, but probably
should be feature-gated forever (if possible).

ignore-pretty in test/run-pass/pushpop-unsafe-okay.rs

src/libsyntax/ast.rs

@@ -810,6 +810,8 @@ pub type SpannedIdent = Spanned<Ident>;
 pub enum BlockCheckMode {
     DefaultBlock,
     UnsafeBlock(UnsafeSource),
+    PushUnsafeBlock(UnsafeSource),
+    PopUnsafeBlock(UnsafeSource),
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]