second draft: add Bitfields to D

[Walter Bright]

I listed these 3 use cases in the second draft, and it seems we are mostly in 
agreement.

Using bit fields to reduce memory consumption, and to be compatible with C code, 
is handled by default nicely with the proposal.

Conformance to an externally imposed layout sometimes is necessary, but it is 
much less common. It is almost always easily done with a minor bit of attention. 
The worst case is writing a shift/mask accessor function, very easy to do. I 
suspect these workarounds are even less effort than reading the spec on how to 
use special syntax for it. Nobody is obliged to use std.bitmanip.bitfield to get 
the job done.

I can help with any externally defined format anyone is having difficulty with.