std.bitmanip improvements

[Marco Nembrini]

Hi,
I was looking into the BitArray part of std.bitmanip to implement 
various data compression routines like Huffmann, Lempel-Ziv etc.. 
and noticed that it could use some improvements, like more 
convenient constructors and functions (for ex. see issue 4717 ). 
How would I go about submitting those changes? Should I put all 
the changes in one big pull request or split it up according to 
the different enhancement requests on Bugzilla?
Also is anyone else working on this module or is there something 
in other parts of the library that I should be aware of?

Cheers,
Marco