Remove the unused obsolete enum_discrim type.
Remove unused std::routine
auto merge of #10677 : jld/rust/type-enum-discrim-rm, r=alexcrichton
auto merge of #10679 : alexcrichton/rust/no-routine, r=pcwalton
Shink NodeId, CrateNum, Name and Mrk down to 32 bits on x64.
Add an iterator for receiving messages from GenericPorts
auto merge of #10670 : eddyb/rust/node-u32, r=alexcrichton

### Rationale
There is no reason to support more than 2³² nodes or names at this moment, as compiling something that big (even without considering the quadratic space usage of some analysis passes) would take at least **64GB**.
Meanwhile, some can't (or barely can) compile rustc because it requires almost **1.5GB**.

### Potential problems
Can someone confirm this doesn't affect metadata (de)serialization? I can't tell myself, I know nothing about it.

### Results
Some structures have a size reduction of 25% to 50%: [before](https://gist.github.com/luqmana/3a82a51fa9c86d9191fa) - [after](https://gist.github.com/eddyb/5a75f8973d3d8018afd3).
Sadly, there isn't a massive change in the memory used for compiling stage2 librustc (it doesn't go over **1.4GB** as [before](http://huonw.github.io/isrustfastyet/mem/), but I can barely see the difference).
However, my own testcase (previously peaking at **1.6GB** in typeck) shows a reduction of **200**-**400MB**.
auto merge of #10688 : bjz/rust/recv_iter, r=brson

I've noticed I use this pattern quite a bit:

~~~rust
do spawn {
    loop {
        match port.try_recv() {
            Some(x) => ...,
            None => ...,
        }
    }
}
~~~

The `RecvIterator`, returned from a default `recv_iter` method on the `GenericPort` trait, allows you to reduce this down to:

~~~rust
do spawn {
    for x in port.recv_iter() {
        ...
    }
}
~~~

As demonstrated in the tests, you can also access the port from within the `for` block for further `recv`ing and `peek`ing with no borrow errors, which is quite nice.