Use Block RAM for input/output shift registers on ice40.

[jkiv]

Convert input/output data storage from registers to block RAM.

We're currently using about 700 bits worth of shift registers... Maybe we can squeeze some more compute power out of certain devices by using block RAM instead.

[jkiv]

Eh, not clear from datasheet if this is supported at a high level...

[jkiv]

It's probably easy enough to manage with address pointers. Might be worthwhile to try.

[jkiv]

Only really makes sense for the message body. Would need to detect/select/redirect those bytes to RAM.

[jkiv]

Each ice40 device has between 8 and 32 256x16 (4kbits) RAM blocks.

Currently using two for K[t] (32-bits). For M[t], could use another two, leaving 4 to 28 remaining.