By instantiating the LUTRAM cell directly, we avoid a trip through
altsyncram, which speeds up Quartus synthesis time. This also gives
a little more flexibility, as Yosys can build RAMs out of individual
32x1 LUTRAM cells.
While working on this, I discovered that the mem_init0 parameter of
<family>_mlab_cell gets ignored by Quartus.
By operating at a layer of abstraction over the rather clumsy Intel primitives,
we can avoid special hacks like `dffinit -highlow` in favour of simple techmapping.
This also makes the primitives much easier to manipulate, and more descriptive
(no more cyclonev_lcell_comb to mean anything from a LUT2 to a LUT6).
Dan Ravensloft
Claire Wolf