Allow the structure to be initialized as all zeros

[jamesmunns]

This PR changes the structure initialization so that all fields can always initially be zero, and therefore reside entirely within .bss.

Notably, this changes the last tracking pointer to be initially zero, and tweaks the write commit logic to handle this gracefully.

Closes #43

With this code:

static BUFFY: BBBuffer<U4096> = BBBuffer( ConstBBBuffer::new() );

Before:

   text	   data	    bss	    dec	    hex	filename
  18934	   4136	      4	  23074	   5a22	timer-uart

After:

   text	   data	    bss	    dec	    hex	filename
  18926	     16	   4120	  23062	   5a16	timer-uart

CC jonas-schievink/rubble#100

[jamesmunns]

@jonas-schievink @Chocol4te could you please confirm this addresses the issue for you as well? If so I'll merge this and release it as 0.4.1

[fmckeogh]

Just tested with an nRF52810, confirmed that it is in bss with arm-none-eabi-size :)