Switch to purely namespaced enums

This breaks code that referred to variant names in the same namespace as
their enum. Reexport the variants in the old location or alter code to
refer to the new locations:

```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
=>
```
pub use self::Foo::{A, B};

pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
or
```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = Foo::A;
}
```

[breaking-change]

src/etc/unicode.py

@@ -391,6 +391,7 @@ def emit_conversions_module(f, lowerupper, upperlower):
 def emit_grapheme_module(f, grapheme_table, grapheme_cats):
     f.write("""pub mod grapheme {
     use core::slice::SlicePrelude;
+    pub use self::GraphemeCat::*;
     use core::slice;
 
     #[allow(non_camel_case_types)]