auto merge of #5320 : apasel422/rust/metaderive, r=graydon

This is the first in a series of patches I'm working on to clean up the code related to `deriving`. This patch allows

```
#[deriving_eq]
#[deriving_iter_bytes]
#[deriving_clone]
struct Foo { bar: uint }
```
to be replaced with:
```
#[deriving(Eq, IterBytes, Clone)]
struct Foo { bar: uint }
```
It leaves the old attributes alone for the time being.

Eventually I'd like to incorporate the new closest-match-suggestion infrastructure for mistyped trait names, and also pass the sub-attributes to the deriving code, so that the following will be possible:
```
#[deriving(TotalOrd(qux, bar))]
struct Foo { bar: uint, baz: char, qux: int }
```
This says to derive an `impl` in which the objects' `qux` fields are compared first, followed by `bar`, while `baz` is ignored in the comparison. If no fields are specified explicitly, all fields will be compared in the order they're defined in the `struct`. This might also be useful for `Eq`. Coming soon.

src/libsyntax/ext/base.rs

@@ -148,6 +148,9 @@ pub fn syntax_expander_table() -> SyntaxEnv {
     syntax_expanders.insert(@~"log_syntax",
                             builtin_normal_tt(
                                 ext::log_syntax::expand_syntax_ext));
+    syntax_expanders.insert(@~"deriving",
+                            @SE(ItemDecorator(
+                                ext::deriving::expand_meta_deriving)));
     syntax_expanders.insert(@~"deriving_eq",
                             @SE(ItemDecorator(
                                 ext::deriving::expand_deriving_eq)));