Add align_offset intrinsic

[oli-obk]

see rust-lang/rfcs#2043 for details and the plan towards stabilization (reexport in core::mem via various convenience functions)

as per @scottmcm 's comment, this is just the intrinsic (which is obviously unstable).

[rust-highfive]

r? @aturon

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[scottmcm]

nit: Should this be unsafe, for consistency with the real intrinsic?

[scottmcm]

Consider adding an # Examples doctest to demo a few simple cases.

[kennytm]

doc test failed.

[00:56:48] failures:
[00:56:48] 
[00:56:48] ---- intrinsics.rs - intrinsics::align_offset (line 1361) stdout ----
[00:56:48] 	error: use of unstable library feature 'core_intrinsics': intrinsics are unlikely to ever be stabilized, instead they should be used through stabilized interfaces in the rest of the standard library (see issue #0)
[00:56:48]  --> intrinsics.rs:9:14
[00:56:48]   |
[00:56:48] 9 | let offset = align_offset(ptr as *const (), align_of::<u16>());
[00:56:48]   |              ^^^^^^^^^^^^
[00:56:48]   |
[00:56:48]   = help: add #![feature(core_intrinsics)] to the crate attributes to enable
[00:56:48] 
[00:56:48] error: use of unstable library feature 'core_intrinsics': intrinsics are unlikely to ever be stabilized, instead they should be used through stabilized interfaces in the rest of the standard library (see issue #0)
[00:56:48]  --> intrinsics.rs:5:5
[00:56:48]   |
[00:56:48] 5 | use std::intrinsics::align_offset;
[00:56:48]   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[00:56:48]   |
[00:56:48]   = help: add #![feature(core_intrinsics)] to the crate attributes to enable
[00:56:48] 
[00:56:48] error[E0308]: mismatched types
[00:56:48]   --> intrinsics.rs:11:30
[00:56:48]    |
[00:56:48] 11 |     let u16_ptr = ptr.offset(offset) as *const u16;
[00:56:48]    |                              ^^^^^^ expected isize, found usize
[00:56:48] 
[00:56:48] thread 'rustc' panicked at 'couldn't compile the test', /checkout/src/librustdoc/test.rs:280:12
[00:56:48] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:56:48] 
[00:56:48] 
[00:56:48] failures:
[00:56:48]     intrinsics.rs - intrinsics::align_offset (line 1361)
[00:56:48] 
[00:56:48] test result: FAILED. 1231 passed; 1 failed; 1 ignored; 0 measured; 0 filtered out
[00:56:48] 
[00:56:48] error: test failed, to rerun pass '--doc'

[oli-obk]

ping @aturon

[aturon]

@bors: r+

[bors]

📌 Commit be96ad2 has been approved by aturon

[bors]

⌛ Testing commit be96ad2 with merge c66e7fa...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: aturon
Pushing c66e7fa to master...