Return an error if there are two enums with the same name

[thomasetter]

This fixes the issue that two enums with the same name cause the "second" one to overwrite the "first" one.

Currently, looking at the output of target/wasm32-unknown-unknown/wbg-tmp-wasm-<some-hex-characters>.wasm/wasm-bindgen-test.js generated with

WASM_BINDGEN_SPLIT_LINKED_MODULES=1 cargo test --target wasm32-unknown-unknown

the part with the enums (was at line 4849 for me) looks like this:

**
*/
module.exports.Color = Object.freeze({ Green:0,"0":"Green",Yellow:1,"1":"Yellow",Red:2,"2":"Red", });
/**
*/
module.exports.JsRenamedEnum = Object.freeze({ A:10,"10":"A",B:20,"20":"B", });
/**
*/
module.exports.EnumArrayElement = Object.freeze({ Unit:0,"0":"Unit", });
/**
* annotated enum type
*/
module.exports.AnnotatedEnum = Object.freeze({
/**
* annotated enum variant 1
*/
Variant1:0,"0":"Variant1",
/**
* annotated enum variant 2
*/
Variant2:1,"1":"Variant2", });
/**
*/
module.exports.Color = Object.freeze({ Green:0,"0":"Green",Yellow:1,"1":"Yellow",Red:2,"2":"Red", });
/**
*/
module.exports.ColorWithCustomValues = Object.freeze({ Green:21,"21":"Green",Yellow:34,"34":"Yellow",Red:2,"2":"Red", });
/**
*/
module.exports.MyEnum = Object.freeze({ One:1,"1":"One",Two:2,"2":"Two", });

The second assignment module.exports.Color = overwrites the first one, causing the first one to have no effect.

The reason this was not captured by the tests is that the two enums were identical in shape (same unit keys/values).