improved buffer API to support newly added fixed_buffer<size_t N>
improved buffer API to support newly added fixed_buffer<size_t N>

this fixed_buffer is meant to be used when you need a x0::buffer object
but want to have the internal storage already allocated inside of
x0::buffer's object space (e.g. on the stack).

xzero/x0,xzero/x0,xzero/x0,christianparpart/x0,christianparpart/x0,christianparpart/x0,christianparpart/x0,xzero/x0,christianparpart/x0,christianparpart/x0,xzero/x0,christianparpart/x0,xzero/x0