wrapper for the parsing function parse

parse takes an input of type I
and returns either an error
or a result of type O and a rest of type R

nom.parsers -- namespace for parser combinators and parsers built with nom

the result of parsing, if successful

rest = the rest of the input that was not consumed by the parser
out = the result of parsing, e.g. a concrete syntax tree

short hand for creating a Parser via
given function `f`.

the result of Parser.call()