These are exactly the same as `print!` and `println!` except that
they write to stderr instead of stdout.  Issue rust-lang#39228.

 * Factor out the nigh-identical bodies of `_print` and `_eprint` to a helper
   function `print_to` (I was sorely tempted to call it `_doprnt`).
 * Update the issue number for the unstable `eprint` feature.
 * Add entries to the "unstable book" for `eprint` and `eprint_internal`.
 * Style corrections to the documentation.

Add `eprint!` and `eprintln!` macros to the prelude.

These are exactly the same as `print!` and `println!` except that they write to stderr instead of stdout.  Issues rust-lang#39228 and rust-lang#40528; previous PR rust-lang#39229; accepted RFC rust-lang/rfcs#1869; proposed revision to The Book rust-lang/book#615.

I have _not_ revised this any since the original submission; I will do that later this week.  I wanted to get this PR in place since it's been quite a while since the RFC was merged.

Known outstanding review comments:

* [x] @steveklabnik requested a new chapter for the unstable version of The Book -- please see if the proposed revisions to the second edition cover it.
* [x] @nodakai asked if it were possible to merge the internal methods `_print` and `_eprint` - not completely, since they both refer to different internal globals which we don't want to expose, but I will see if some duplication can be factored out.

Please let me know if I missed anything.

More Queries for Crate Metadata

This covers a little bit of clean up and the following parts of rust-lang#41417:
* `fn item_attrs(&self, def_id: DefId) -> Vec<ast::Attribute>;`
* `fn fn_arg_names(&self, did: DefId) -> Vec<ast::Name>;`
* `fn trait_of_item(&self, def_id: DefId) -> Option<DefId>;`
* `fn impl_parent(&self, impl_def_id: DefId) -> Option<DefId>;`
* ` fn is_foreign_item(&self, did: DefId) -> bool;`
* `fn is_exported_symbol(&self, def_id: DefId) -> bool;`

r? @nikomatsakis

…ookup, r=nikomatsakis

ICH: Handle case of removed FileMaps.

This PR fixes a bug introduced in rust-lang#41709 where removing a source file between compilation sessions would cause an ICE:
https://travis-ci.org/rust-icci/crossbeam/jobs/230582234#L633

r? @nikomatsakis

…ister

compiletest: force GDB to print values in the Rust format.

Based on @nodakai's rust-lang#40557 (comment) and @infinity0's rust-lang#40557 (comment).
Fixes rust-lang#40557.

Remove debug message

It was added by me in rust-lang#41293. Sorry about that!

Thanks goes to @alexbool for finding it.