str: Add better tests for string slice's Chars iterator

Test using for ch s.chars() { black_box(ch) } to have a test that should
force the iterator to run its full decoding computations.
core::str: Implement Chars iterator using slice::Items

Re-use the vector iterator to implement the chars iterator.

The iterator uses our guarantee that the string contains valid UTF-8,
but its only unsafe code is transmuting the decoded u32 into char.
Add more tests for str Chars iterator

Test iterating (decoding) every codepoint.
librustc: Emit enum variant constructor at callsite instead of via a call to a function.
librustc: Emit tuple struct constructor at callsite instead of via a call to a function.
librustc: Only emit constructor functions as necessary.
librustc: Set enum discriminant only after field translation.
Ignore one test.
Clarify str Chars iterator implementation

Thanks to comments from @huonw, clarify decoding details and use
statics for important constants for UTF-8 decoding. Convert some magic
numbers scattered in the same file to use the statics too.
Write multibyte case for str Chars iterator in-line

Thanks to comments from @alexcrichton, write the next/next_back function
bodies without nested functions in a more top-to-bottom flow style.

Also improve comment style and motivate the unsafe blocks with comments.
auto merge of #15765 : luqmana/rust/iec, r=pcwalton

Fixes #15400.
Simplify str CharOffsets iterator

Only one uint is needed to keep track of the offset from the original
full string.
auto merge of #15638 : blake2-ppc/rust/ptr-arithmetic-chars, r=huonw

Reimplement the string slice's `Iterator<char>` by wrapping the already efficient
slice iterator.

The iterator uses our guarantee that the string contains valid UTF-8, but its only unsafe
code is transmuting the decoded `u32` into `char`.

Benchmarks suggest that the runtime of `Chars` benchmarks are reduced by up to 30%,
runtime of `Chars` reversed reduced by up to 60%.

```
BEFORE
test str::bench::char_indicesator                          ... bench:       124 ns/iter (+/- 1)
test str::bench::char_indicesator_rev                      ... bench:       188 ns/iter (+/- 9)
test str::bench::char_iterator                             ... bench:       122 ns/iter (+/- 2)
test str::bench::char_iterator_ascii                       ... bench:       302 ns/iter (+/- 41)
test str::bench::char_iterator_for                         ... bench:       123 ns/iter (+/- 4)
test str::bench::char_iterator_rev                         ... bench:       189 ns/iter (+/- 14)
test str::bench::char_iterator_rev_for                     ... bench:       177 ns/iter (+/- 4)

AFTER
test str::bench::char_indicesator                          ... bench:        85 ns/iter (+/- 3)
test str::bench::char_indicesator_rev                      ... bench:        82 ns/iter (+/- 2)
test str::bench::char_iterator                             ... bench:       100 ns/iter (+/- 3)
test str::bench::char_iterator_ascii                       ... bench:       317 ns/iter (+/- 3)
test str::bench::char_iterator_for                         ... bench:        86 ns/iter (+/- 2)
test str::bench::char_iterator_rev                         ... bench:        80 ns/iter (+/- 6)
test str::bench::char_iterator_rev_for                     ... bench:        68 ns/iter (+/- 0)
```

Note: Branch name is no longer indicative of the implementation.
Implement new mod import sugar

Implements RFC #168.