Allow variables in Task names

[David-OConnor]

Desired behavior:

#[cfg(feature = "h7")]
const DMA1_2: Interrupt = Interrupt::DMA1_STR2;
#[cfg(feature = "g4")]
const DMA1_2: Interrupt = Interrupt::DMA1_CH2;

#[task binds = DMA1_2, ...

Follow-up from Element thread with Korken. Sounds possible via supporting CFG in the binds argument. Posting here due to it being easier to catalog the request.

Currently, the Task (and other) RTIC macros don't allow dynamic code; they require hard-coded names.

#664 is related.