Explicit alignment in the type system

See http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1507.htm and http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1397.htm

[Aatch]

Marking as maturity 4 since alignment is useful, but not required. Adding it should be backwards compatible unless it (for some reason) changes alignment rules.

[emberian]

Nominating for production ready

[emberian]

This would probably be implemented using intrinsics and attributes, rather than requiring language changes, I think.

[graydon]

This is tangled up with the question of constant expressions. #5551

[graydon]

accepted for feature-complete milestone

[thestinger]

I think we would need the ability to add attributes to variable declarations for this to be practical.

[nikomatsakis]

Seems like being able to specify specific alignments on struct types would be sufficient.

[pnkfelix]

Not a 1.0 blocker, P-low. (We think it sufficent to be able to put such attributes on struct fields and/or struct items themselves; if you need a stack local aligned, wrap it in such a struct.)

[thestinger]

#14006 implements proper alignment handling for heap allocations, so it's a lot more feasible to implement this as a property of types now.

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#282