Make BlobTyBuilder a simple function instead of a builder struct

[fitzgen]

BlobTyBuilder is a struct with a new method for construction and a build method to complete the builder. But it doesn't have any methods to configure the builder or anything like that. So every use is like BlobTyBuilder::new(..).build(). This would be better as a simple function blob(..), since we aren't using it as a builder struct at all.

https://github.com/rust-lang-nursery/rust-bindgen/blob/723e93d1267e58cb1aafd04d221a0dff17e212d9/src/codegen/helpers.rs#L63

[highfive]

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[MaloJaffre]

I'll try to open a PR shortly.
@highfive: assign me