Bin trie nits

[lrvideckis]

a couple changes:

• change type of sub_sz away from isize/usize
• added pub type T = u32; so now if you want only 32 bits, it should be twice as fast & half the memory as previously with usize
• if let Some() = .. syntax similar to https://github.com/programming-team-code/programming_team_code_rust/pull/84