Add `HygieneData::default_transparency`.
Add `HygieneData::default_transparency`.

Also use `HygieneData::expn_info` in an appropriate place.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust