Use a binding's field identifier to produce more descriptive output.
Fall back to a binding's identifier if a field identifier is not
available.
Additionally, make the privatize.rs compiletest more specific and
wrap the identifier in backticks to aid readability.
It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
* Sections like `[dependencies.foo]` can be entries in a `[dependencies]`
section with the `{key = value}` syntax.
* Per-target dependencies can be expressed with more general `cfg(…)`
conditions instead of exact target triples:
https://github.com/rust-lang/cargo/pull/2328
