Rewrite startsWith and endsWith to accept consumers #165
Description
These functions currently only accept sequences. If they were changed to expect consumers (as are in development as of writing in this branch) then the current use cases would still be supported with little or no syntax changes, but the functions would also become more flexible.
Note that for a reasonably performant of endsWith done this way, consumers would need to be made bidirectional. (Which they aren't currently.) I think this is possible without a lot of changes, though! Just something for me to keep in mind as I work in the aforementioned branch I suppose.