Add std::num::Saturating

Saturating is an implementation of saturating math operations (at the
moment just add and sub) for integral types.
Update Iterator impls to use Saturating

Replace hand-rolled saturation math with calls to Saturating.

Fix one impl that didn't use saturating math.
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 #8323 : kballard/rust/saturating, r=thestinger

Implement saturating math in `std::num::Saturating` and use it for `Iterator` impls
std: adjust str::test_add so that the macro expands to all 3 items (#8012).

Closes #3682.
testsuite: remove 2 very outdated testcases; functionality is tested by other class-*.rs tests.

Fixes #7305, #7307.
testsuite: remove incorrect section of 2 test cases. (Also, &const is disappearing.)

Fixes #7304.
testsuite: add explanation to a Note, and remove duplicated code.

Fixes #7302.
auto merge of #8305 : huonw/rust/triage-fixes, r=cmr

The two deletions are because the test cases are very old (still using `class` and modes!), and, as far as I can tell (since they are so old), the areas they test are well tested by other rpass tests.
option.get -> option.unwrap
std: add result.map_move, result.map_err_move
Revert "Re-enable rusti tests"

This reverts commit d5de801cc1b4a3147dfd5a29a3d0e129ee0fe783.
core: option.map_consume -> option.map_move
std: removed option.take_map{,_default}