Querying a type's alignment

[HertzDevil]

In #7773 there are proposals to control a type's alignment, but there is still no way to query it. Therefore, I suggest the addition of alignof and instance_alignof, which are usable wherever sizeof and instance_sizeof also are:

struct Foo
  @x : Int8
  @y : Int32
  @z : UInt16
end

alignof(Foo) # => 4

@[Extern]
@[Packed]
struct Foo2
  @x : Int8
  @y : Int32
  @z : UInt16
end

alignof(Foo2) # => 1

class Bar
  @x : Int64
end

instance_alignof(Bar) # => 8

class Bar2
end

instance_alignof(Bar2) # => 4

It would become possible to write things like:

@[Align(instance_alignof(Bar))]
buf = uninitialized UInt8[instance_sizeof(Bar)]
bar = buf.to_unsafe.as(Bar)

Also this would provide a means to implement type-aware allocators in native Crystal that align their pointers correctly. The benefits may not be immediately apparent as nothing could have a larger alignment than Int128 (16 bytes) right now, but this could change if eventually #3057 arrives.

The alignment for a single instance variable of a type can be implemented as alignof(Foo, @z). Normally this is identical to alignof(typeof((x = uninitialized Foo; x.@z))), but a separate syntax is necessary because #7773 also allows @[Align] to be applied to an individual instance variable.