support no_std mode

[sarah-ek]

switch to core and alloc instead of std. also use hashbrown hashmap and hashset instead of the one from std

[Andlon]

Thanks @sarah-ek, that's great. Seems like you're missing the actual hashbrown dep in Cargo.toml?

Also, is the motivation for using hashbrown simply that it supports no-std? My understanding is that HashMap/HashSet from std are not no-std compatible because they need a source of randomness for security purposes. Do you know how hashbrown handles this? I couldn't dig up docs on how hashbrown differs for the no-std case. I can't quite imagine matrixcompare being used in a place where this could become an issue, but I don't know. Do you have any thoughts on this?

[sarah-ek]

the randomness is there to prevent DOS attacks, which seems unlikely to me with sparse matrices?