SimdSignal of only 8 bits in length or so, but having a **different**
style of PartitionPoints, with no padding this time)
+take signal a, of 16 bits, each bit being numbered in hexadecimal:
+
+ | | |
+ AfAeAdAc AbAaA9A8 A7A6A5A4 A3A2A1A0
+
+and take a slice a[0:2] to create 3-bit values, where padding is
+specified by "x", at each elwid:
+
+ elwid | | |
+ 0b00 AfAeAdAc AbAaA9A8 A7A6A5A4 A3A2A1A0
+
Illustrating the case where a Sliced (fixed element width) SimdSignal
is added to one which has variable-length elements that take up the
entirety of the partition (overall fixed width):