Expand the trees API

[molpopgen]

• [x] TreeSequence list of samples
• [x] TreeSequence::new method for a list of samples (useful when node flags not used).
• [x] TreeSequence should error out when tables are not indexed.
• [x] Calculate number of trees when initializing a tree sequence
• [x] TreeSequence init should validate tables by default.
• [x] Add TreeSequence::simplify.
• [x] Preorder node traversal of a tree
• [x] The various "node iterators" a la tskit-rust.
• [x] Replace panic! and (most) assert! with proper error propagation to ForrusttsError.
• [x] Add TreesError as a from to ForrusttsErrror.
• [x] TreeSeq::new_with_samples should validate sample lists.