* tilelink2 Narrower: support widenening and narrowing on all channels
Be extra careful with the mask transformations
We need to make sure that narrowing or widening do not cause a loss
of information about the operation. The addr_hi+(mask|addr_lo) conversions
are now 1-1, except on D, which should not matter.
* tilelink2 SRAM: work around firrtl SeqMem bug
* tilelink2 WidthWidget: renamed from Narrower (it now converts both ways)
* tilelink2 mask: fix an issue with width=1 data buses
