optimize field ordering by grouping m*2^n-sized fields with equivalently aligned ones
```rust
use std::ptr::addr_of;
use std::mem;
struct Foo {
word: u32,
byte: u8,
ary: [u8; 4]
}
fn main() {
let foo: Foo = unsafe { mem::zeroed() };
println!("base: {:p}\nword: {:p}\nbyte: {:p}\nary: {:p}", &foo, addr_of!(foo.word), addr_of!(foo.byte), addr_of!(foo.ary));
}
```
prints
```
base: 0x7fffc1a8a668
word: 0x7fffc1a8a668
byte: 0x7fffc1a8a66c
ary: 0x7fffc1a8a66d
```
I.e. the `u8` in the middle causes the array to sit at an odd offset, which might prevent optimizations, especially on architectures where unaligned loads are costly.
Note that this will make field ordering niche-dependent, i.e. a `Bar<T>` with `T=char` and `T=u32` may result in different field order, this may break some code that makes invalid assumptions about `repr(Rust)` types.
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