Attempt at better modularity of parsing, w/r/t paths

[susan-garry]

This is an attempt to delegate more of the parsing of GFA path to gfaline.rs, along with other GFA lines, as opposed to parse.rs.

Because gfaline::Path stores references to strings and parsing of paths must be deferred until after steps are parsed, parse.rs still stores entire path lines to be parsed later, but now makes use of gfaline::parse_line much in the same way that it already did for links. Parts of the code are now more elegant, parts of them less so, and I think the only thing this really accomplished is satisfying the compiler's desire that all fields of gfaline::Path are read from.