Move some primitives to prims/string

add a few string primitives
(only one test more, sorry)
Fix list_tail
Tho not working, broaden scope of input for regex-es
Allow more arguments for the read-*line primitives
Allow more format chars

(necessary for shootout benches #83)
i/o-prims and StringPort fix

 * open-input-string
 * open-output-string -> open-output-bytes (alias for now)
 * read-bytes-avail! -> read-bytes! (alias for now)
 * write-bytes-avail -> write-bytes (alias for now)
 * Performance fix in W_StringOutputPort and W_StringInputPort
Shortcuts in read/write-bytes
expose `eof`