Auto merge of #257 - fitzgen:more-logging-and-asserts, r=emilio
Auto merge of #257 - fitzgen:more-logging-and-asserts, r=emilio

Add more logging for item insertion and asserts

This adds `debug!` logging for all paths that insert items into
`BindgenContext::items`, and adds some `debug_assert!`s against dangling
`ItemId`s.

r? @emilio

emilio/rust-bindgen,jdub/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,rust-lang/rust-bindgen