Auto merge of #741 - tmfink:feature-699-constified-enum-module, r=emilio

Feature 699 constified enum module

This is a work in progress for issue #699 that adds the `--constified-enum-module` option to bindgen.

@emilio, could you give me some guidance on fixing the uses of the enum variant types? In the example below, `foo` should be replaced with `foo::Type`. I'm not sure of the proper way to rename `Item`s after the structures have been defined. My initial thought was to redefine the `CodeGenerator` trait to take a mutable reference to `item`, but that will not work because of the borrow checker. Thoughts?

Todo:
- [x] put constified enum variants in a `mod`
- [x] ensure references to constified enum `foo` are changed to `foo::Type`
- [x] handle `typedef` enums

-----

Given the input header `tests/headers/constify-module-enums.h`:
~~~c
// bindgen-flags: --constified-enum-module foo

enum foo {
  THIS,
  SHOULD_BE,
  A_CONSTANT,
};

struct bar {
  enum foo this_should_work;
};
~~~

`$ cargo run -- tests/headers/constify-module-enums.h --constified-enum-module foo --no-layout-tests` will output:
~~~rust
/* automatically generated by rust-bindgen */

pub mod foo {
    pub type Type = ::std::os::raw::c_uint;
    pub const THIS: Type = 0;
    pub const SHOULD_BE: Type = 1;
    pub const A_CONSTANT: Type = 2;
}
#[repr(C)]
#[derive(Debug, Copy)]
pub struct bar {
    pub this_should_work: foo,
}
impl Clone for bar {
    fn clone(&self) -> Self { *self }
}
~~~

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