Use slice representation for child crate maps
Use slice representation for module entries in CrateMap

Relaxe lifetime of CrateMap as well.
Use &'self str instead of raw char pointer in ModEntry
Add code for older crate map versions, bumped crate map version number
Implement feature-gating for the compiler

A few features are now hidden behind various #[feature(...)] directives. These
include struct-like enum variants, glob imports, and macro_rules! invocations.

Closes #9304
Closes #9305
Closes #9306
Closes #9331
auto merge of #9738 : catamorphism/rust/rustpkg-test-docs-need-to-submit, r=alexcrichton

r? anybody Talk about `rustpkg test` in the tutorial, and update its usage message.

@steveklabnik, it would be great if you could look this over :-)
Fix a typo in std::ops documentation
Clearer error message for external trait and type

The old error message implied that external traits could never
be implemented locally.
auto merge of #9682 : skade/rust/clearer-error-message-for-external-type-and-trait, r=alexcrichton

The old error message implied that external traits could never
be implemented locally.
get_crate_map returns an Option
auto merge of #9593 : fhahn/rust/logging-unsafe-removal, r=alexcrichton

This pull request changes to memory layout of the `CrateMap` struct to use static slices instead of raw pointers. Most of the discussion took place [here](https://github.com/fhahn/rust/commit/63b5975efa10af7df3df0a50d8d7f9c8d5bcf9e9#L1R92) .

The memory layout of CrateMap changed, without bumping the version number in the struct. Another, more backward compatible, solution would be to keep the old code and increase the version number in the new struct. On the other hand, the `annihilate_fn` pointer was removed without bumping the version number recently.

At the moment, the stage0 compiler does not use the new memory layout, which would lead the segfaults during stage0 compilation, so I've added a dummy `iter_crate_map` function for stage0, which does nothing. Again, this could be avoided if we'd bump the version number in the struct and keep the old code.

I'd like to use a normal `for` loop [here](https://github.com/fhahn/rust/compare/logging-unsafe-removal?expand=1#L1R109), 

        for child in children.iter() {
            do_iter_crate_map(child, |x| f(x), visited);
        }


but for some reason this only yields `error: unresolved enum variant, struct or const 'Some'` and I have no idea why.
auto merge of #9741 : catamorphism/rust/rustpkg-remotes, r=cmr

r? @cmr Closes #9193
Add appropriate #[feature] directives to tests
auto merge of #9703 : alexcrichton/rust/compiler-features, r=cmr

This implements the necessary logic for gating particular features off by default in the compiler. There are a number of issues which have been wanting this form of mechanism, and this initially gates features which we have open issues for.

Additionally, this should unblock #9255
rustpkg: Make checked-out source files read-only, and overhaul where temporary files are stored

rustpkg now makes source files that it checks out automatically read-only, and stores
them under build/.

Also, refactored the `PkgSrc` type to keep track of separate source and destination
workspaces, as well as to have a `build_workspace` method that returns the workspace
to put temporary files in (usually the source, sometimes the destination -- see
comments for more details).

Closes #6480