std: Stabilize APIs for the 1.8 release

This commit is the result of the FCPs ending for the 1.8 release cycle for both
the libs and the lang suteams. The full list of changes are:

Stabilized

* `braced_empty_structs`
* `augmented_assignments`
* `str::encode_utf16` - renamed from `utf16_units`
* `str::EncodeUtf16` - renamed from `Utf16Units`
* `Ref::map`
* `RefMut::map`
* `ptr::drop_in_place`
* `time::Instant`
* `time::SystemTime`
* `{Instant,SystemTime}::now`
* `{Instant,SystemTime}::duration_since` - renamed from `duration_from_earlier`
* `{Instant,SystemTime}::elapsed`
* Various `Add`/`Sub` impls for `Time` and `SystemTime`
* `SystemTimeError`
* `SystemTimeError::duration`
* Various impls for `SystemTimeError`
* `UNIX_EPOCH`
* `ops::{Add,Sub,Mul,Div,Rem,BitAnd,BitOr,BitXor,Shl,Shr}Assign`

Deprecated

* Scoped TLS (the `scoped_thread_local!` macro)
* `Ref::filter_map`
* `RefMut::filter_map`
* `RwLockReadGuard::map`
* `RwLockWriteGuard::map`
* `Condvar::wait_timeout_with`

Closes rust-lang#27714
Closes rust-lang#27715
Closes rust-lang#27746
Closes rust-lang#27748
Closes rust-lang#27908
Closes rust-lang#29866

reference.md

@@ -1141,15 +1141,13 @@ the list of fields entirely. Such a struct implicitly defines a constant of
 its type with the same name. For example:
 
 ```
-# #![feature(braced_empty_structs)]
 struct Cookie;
 let c = [Cookie, Cookie {}, Cookie, Cookie {}];
 ```
 
 is equivalent to
 
 ```
-# #![feature(braced_empty_structs)]
 struct Cookie {}
 const Cookie: Cookie = Cookie {};
 let c = [Cookie, Cookie {}, Cookie, Cookie {}];
@@ -2385,7 +2383,6 @@ The currently implemented features of the reference compiler are:
                               terms of encapsulation).
 * - `default_type_parameter_fallback` - Allows type parameter defaults to
                                         influence type inference.
-* - `braced_empty_structs` - Allows use of empty structs and enum variants with braces.
 
 * - `stmt_expr_attributes` - Allows attributes on expressions and
                              non-item statements.