Implement `#[deriving]` meta-attribute

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.