Consider: T:Copy could be bound to ~T, which is not implicitly copyable.

The lookups for these items in external crates currently cause repeated
decoding of the EBML metadata, which is pretty slow. Adding caches to
avoid the repeated decoding reduces the time required for the type
checking of librustc by about 25%.

This fixes the strange random crashes in compile-fail tests.

This reverts commit 96cd61ad.

Conflicts:
	src/librustc/driver/driver.rs
	src/libstd/str.rs
	src/libsyntax/ext/quote.rs

This almost removes the StringRef wrapper, since all strings are
Equiv-alent now. Removes a lot of `/* bad */ copy *`'s, and converts
several things to be &'static str (the lint table and the intrinsics
table).

There are many instances of .to_managed(), unfortunately.

* Don't return early, so logging is not skipped
* Remove one allocation
* Indent the match statement correctly

We were just looking at the last because we were overwriting ret.

Related to #5062 (same error message), but that test case causes
a different ICE than mentioned there (even without this fix).

Fix a laundry list of warnings involving unused imports that glutted
up compilation output.  There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.

Smallintmap was being populated with sparse keys during ast deserialization.

Impls can implement either zero or one traits; this has been true
more or less since we removed classes. So I got rid of the comments
saying "we should support multiple traits" and changed the code to
make it clear that we don't. This is just cleanup, and doesn't break
any existing tests.

but do not integrate with kindck etc (requires a snapshot first)

Replace all instances of #[auto_*code] with the appropriate #[deriving] attribute
and remove the majority of the actual code, leaving stubs to refer the user to
the new syntax.