Working to impl embedded-dma for SPIM#234

Draft

jamesmunns wants to merge 8 commits intomasterfrom

Commits on Sep 28, 2020

Add spim DMA
James Munns
committed
Fix Spis done peek
James Munns
committed
Take on drop
James Munns
committed

Commits on Sep 29, 2020

Fix SPIS (again?), add `bail` methods, change drop to stop immediately
James Munns
committed
Oops
James Munns
committed
Revert bad change
James Munns
committed

Commits on Oct 6, 2020

Rework flag clearing, switch unwrap to checked for now
James Munns
committed

Commits on Oct 7, 2020

Why doesn't that work?
James Munns
committed