Also, makes the pretty-printer use & instead of @ as much as possible,
which will help with later changes, though in the interim has produced
some... interesting constructs.

Drop is no longer a keyword, removed it from the intro docs.

After getting an ICE trying to use the `Repr` enum from middle::trans::adt (see issue #7527), I tried to implement the missing case for struct-like enum variants in `middle::ty::enum_variants()`. It seems to work now (and passes make check) but there are still some uncertainties that bother me:
+ I'm not sure I did everything, right. Especially getting the variant constructor function from the variant node id is just copied from the tuple-variant case. Someone with more experience in the code base should be able to see rather quickly whether this OK so.
+ It is kind of strange that I could not reproduce the ICE with a smaller test case. The unimplemented code path never seems to be hit in most cases, even when using the exact same `Repr` enum, just with `ty::t` replaced by an opaque pointer. Also, within the `adt` module, `Repr` and matching on it is used multiple times, again without running into problems. Can anyone explain why this is the case? That would be much appreciated. 

Apart from that, I hope this PR is useful.

This is more future-proof

This a followup to #7510. @catamorphism requested a test - so I have created one, but in doing so I noticed some inconsistency in the error messages resulting from referencing nonexistent traits, so I changed the messages to be more consistent.

Change the signature of Iterator.size_hint() to always have a lower bound.

Implement .size_hint() on all remaining iterators (if it differs from the default).

Add size_hint() to the Iterators in libextra and the Iterator in
libsyntax.

Skip deque for the moment, as it's being worked on elsewhere.

Fix an assertion in grow when using add_front.

Speeds up grow (and the functions that use it) by a lot. We retain the vector instead of creating it anew for each grow. 

The struct field hi is removed since it is redundant, and all iterators are updated to use a representation closer to the Deque itself, it should make it work even if deque sizes are not powers of two.

Deque::with_capacity is also implemented, but .reserve() is not yet done.

They simply byte-swap an integer to a specific endian, like the hton* functions in C.

These intrinsics are synthesized, so maybe they should be in another file. But since they are just a single line of code each, based on the bswap intrinsics and aren't really intended for public consumption I thought they would fit in the intrinsics file.

The next step working on this could be to expose a trait / generic function for byteswapping.

c9b9462e r=z0w0
2e65782c r=cmr
20458899 r=thestinger
30fca57f r=huonw

Also marks them with `#[deny(warnings)]` (I think they're both only ever really built past stage1)

disable rusti tests, nothing's landing

We need a reasonably small initial capacity to make Deques faster
for the common case.

The deque is split at the marker lo, or logical index 0. Move the
shortest part (split by lo) when growing. This way add_front is just as
fast as add_back, on average.

The previous implementation of reverse iterators used modulus (%) of
negative indices, which did work but was fragile due to dependency on
the divisor.

The deque is determined by vec self.elts.len(), self.nelts, and self.lo,
and self.hi is calculated from these.

self.hi is just the raw index of element number `self.nelts`

So that deque can be used with IteratorUtil::collect()

Fix some issues with the deque being very slow, keep the same vec around
instead of constructing a new. Move as few elements as possible, so the
self.lo point is not moved after grow.

   [o o o o o|o o o]
       hi...^ ^.... lo

   grows to

   [. . . . .|o o o o o o o o|. . .]
              ^.. lo        ^.. hi

If the deque is append-only, it will result in moving no elements on
grow. If the deque is prepend-only, all will be moved each time.

The bench tests added show big improvements:

Timed using `rust build -O --test extra.rs && ./extra --bench deque`

Old version:

test deque::tests::bench_add_back ... bench: 4976 ns/iter (+/- 9)
test deque::tests::bench_add_front ... bench: 4108 ns/iter (+/- 18)
test deque::tests::bench_grow ... bench: 416964 ns/iter (+/- 4197)
test deque::tests::bench_new ... bench: 408 ns/iter (+/- 12)

With this commit:

test deque::tests::bench_add_back ... bench: 12 ns/iter (+/- 0)
test deque::tests::bench_add_front ... bench: 16 ns/iter (+/- 0)
test deque::tests::bench_grow ... bench: 1515 ns/iter (+/- 30)
test deque::tests::bench_new ... bench: 419 ns/iter (+/- 3)

Add a test that excercises deque growing.

Add bench tests for grow, new, add_back, add_front, to expose how slow
these functions are.

Without this, it will hit the assert in fn grow after 32 consecutive
add_front.

Every iterator adaptor now has an implementation of .size_hint() that
makes sense, except for when the default of (0, None) is correct.

Remove the Option wrapper around the lower bound. None is semantically
the same as Size(0), so there's no point in having a distinction.

Closes #5194

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.

r?

Add the x64 windows target to platform.mk and reorder some headers to fix build on mingw64. There are still some issues with building llvm but this gets us one step closer.

The Base64 package previously had extremely basic functionality. It only
suported the standard encoding character set, didn't support line breaks
and always padded output. This commit makes it significantly more
powerful.

The FromBase64 impl now supports all of the standard variants of Base64.
It ignores newlines,interprets '-' and '_' as well as '+' and '/' and
doesn't require padding. It isn't incredibly pedantic and will
successfully parse strings that are not strictly valid, but I don't
think the extra complexity required to make it accept _only_ valid
strings is worth it.

The ToBase64 trait has been modified such that to_base64 now takes a
base64::Config struct which contains the output format configuration.
This currently includes the selection of character set (standard or
url safe), whether or not to pad and an optional line break width. The
package comes with three static Config structs for the RFC 4648
standard, RFC 4648 url safe and RFC 2045 MIME formats.

The other option for configuring ToBase64 output would be to have one
method with the configuration flags passed and other traits with default
impls for the common cases, but I think that's a little messier.

FromBase64 still kills the task if you pass it invalid input, which isn't
particularly appropriate for a function into which you'll be passing
unvalidated input. Would it be worth changing its signature to return a
Result?