Cleanup to grapefruit-seq in preparation for Cosmo

[mkeeter]

Three relatively standalone commits:

• Add a helper function AuxFlash::get_compressed_blob_streaming to do streaming decompression of a blob in auxflash. We'll use this for both FPGAs on Cosmo.
• Move Spartan7 initialization to a separate crate, since we need to do it on both Grapefruit and Cosmo. This is mostly modeled after drv-ice40-spi-program.
• Remove fmc peripheral from the fmc_demo task, because it's now initialized pre-Hubris.

[hawkw]

This is pretty straightforward and seems good to me! I had one suggestion about the API for the spartan7-spi-program crate, which could make things a bit nicer but is also completely ignorable. :)

[hawkw]

I love the naming "bonus clocks" :D

[mkeeter]

I applied @hawkw 's comments, here's the inter-diff

[hawkw]

I applied @hawkw 's comments, here's the inter-diff

Lovely!