iterator: simplify the `take` implementation
add Extendable to the prelude
iterator: rename `Counter::new` to `count`

to match the convention used by `range`, since `iterator::count` is
already namespaced enough and won't be ambiguous
remove `extra::iter`

This module provided adaptors for the old internal iterator protocol,
but they proved to be quite unreadable and are not generic enough to
handle borrowed pointers well.

Since Rust no longer defines an internal iteration protocol, I don't
think there's going to be any reuse via these adaptors.
vec: avoid `ptrtoint`/`inttoptr` in the iterators

This results in throwing away alias analysis information, because LLVM
does *not* implement reasoning about these conversions yet.

We specialize zero-size types since a `getelementptr` offset will
return us the same pointer, making it broken as a simple counter.
add an intrinsic for inbounds GEP
vec: use `offset_inbounds` for iterators

This allows LLVM to optimize vector iterators to an `getelementptr` and
`icmp` pair, instead of `getelementptr` and *two* comparisons.

Code snippet:

~~~
fn foo(xs: &mut [f64]) {
    for x in xs.mut_iter() {
        *x += 10.0;
    }
}
~~~

LLVM IR at stage0:

~~~
; Function Attrs: noinline uwtable
define void @"_ZN3foo17_68e1b25bca131dba7_0$x2e0E"({ i64, %tydesc*, i8*, i8*, i8 }* nocapture, { double*, i64 }* nocapture) #1 {
"function top level":
  %2 = getelementptr inbounds { double*, i64 }* %1, i64 0, i32 0
  %3 = load double** %2, align 8
  %4 = getelementptr inbounds { double*, i64 }* %1, i64 0, i32 1
  %5 = load i64* %4, align 8
  %6 = ptrtoint double* %3 to i64
  %7 = and i64 %5, -8
  %8 = add i64 %7, %6
  %9 = inttoptr i64 %8 to double*
  %10 = icmp eq double* %3, %9
  %11 = icmp eq double* %3, null
  %or.cond6 = or i1 %10, %11
  br i1 %or.cond6, label %match_case, label %match_else

match_else:                                       ; preds = %"function top level", %match_else
  %12 = phi double* [ %13, %match_else ], [ %3, %"function top level" ]
  %13 = getelementptr double* %12, i64 1
  %14 = load double* %12, align 8
  %15 = fadd double %14, 1.000000e+01
  store double %15, double* %12, align 8
  %16 = icmp eq double* %13, %9
  %17 = icmp eq double* %13, null
  %or.cond = or i1 %16, %17
  br i1 %or.cond, label %match_case, label %match_else

match_case:                                       ; preds = %match_else, %"function top level"
  ret void
}
~~~

Optimized LLVM IR at stage1/stage2:

~~~
; Function Attrs: noinline uwtable
define void @"_ZN3foo17_68e1b25bca131dba7_0$x2e0E"({ i64, %tydesc*, i8*, i8*, i8 }* nocapture, { double*, i64 }* nocapture) #1 {
"function top level":
  %2 = getelementptr inbounds { double*, i64 }* %1, i64 0, i32 0
  %3 = load double** %2, align 8
  %4 = getelementptr inbounds { double*, i64 }* %1, i64 0, i32 1
  %5 = load i64* %4, align 8
  %6 = lshr i64 %5, 3
  %7 = getelementptr inbounds double* %3, i64 %6
  %8 = icmp eq i64 %6, 0
  %9 = icmp eq double* %3, null
  %or.cond6 = or i1 %8, %9
  br i1 %or.cond6, label %match_case, label %match_else

match_else:                                       ; preds = %"function top level", %match_else
  %.sroa.0.0.in7 = phi double* [ %10, %match_else ], [ %3, %"function top level" ]
  %10 = getelementptr inbounds double* %.sroa.0.0.in7, i64 1
  %11 = load double* %.sroa.0.0.in7, align 8
  %12 = fadd double %11, 1.000000e+01
  store double %12, double* %.sroa.0.0.in7, align 8
  %13 = icmp eq double* %10, %7
  br i1 %13, label %match_case, label %match_else

match_case:                                       ; preds = %match_else, %"function top level"
  ret void
}
~~~
auto merge of #8326 : thestinger/rust/iterator, r=alexcrichton

The `extra::iter` module wasn't actually included in `extra.rs` when it was moved from `std`... I assume no one is going to miss it.