mistral: improve ALM packing in LABs

[Ravenslofty]

The current ALM packing code overestimates LAB TD congestion due to not accounting for sharing across ALMs.

Recently, I tried to resolve this by hashing the signal names, but this actually underestimated LAB TD congestion, because the TD-to-GOUT (ALM input) crossbar is not fully populated. The solution to this is to hash signal-and-ALM-input pairs, which still results in a distinct reduction in LAB use for attosoc, from 133 to 123 LABs.

I am a bit concerned about the impact on performance here; while it's still "fine" for small designs, perhaps @gatecat might have an alternative implementation based on the existing unique_input_count code minus some incrementally-updated factor for signal sharing.

This code also does not take into account LD usage, which would need some further thinking.

[gatecat]

I wonder if it makes more sense to do this incrementally - replacing pool with a key->count dict, binds add and increase count as necessary, unbinds decrements count and remove if count is now zero

[Ravenslofty]

I figured you'd have a better way of doing this, but I'm glad the code got the general idea across.