Optimize cst sct3 for large alphabet

    cst_sct3 can now support fast ops on int alphabets.
    
    Methods `degree()`, `lca(v,w)`, `parent()`, `select_child(v,i)`, `sl(v)`
    used a linear scan of at most `degree()` bits. For byte alphabets this
    was just a lookup in 4 64-bit words. For integer alphabets in the
    multi-millions the scan slows down the operations.
    The result of the scan can also be calculated in constant time with
    the help of a rank and select structure. I have now added the
    possibility to specify the rank and select structure, so that the
    operations are still fast on very large alphabets.
    
    The structure is still ABI backward compatible for byte-alphabet
    CSTs. Notice: The ABI changed for integer-alphabets.