Use (Function, Details) in place of the aggregrate DetailedFunction.
It turns out that some tests want to clone and manipulate the function
while the details never change.
Export a single function: parse_functions() which results a vector of
functions parsed from the source string.
Hide the parser and lexer modules. They are not useful to external
clients.
libctonfile -> libreader.
This library will only provide .cton file reading/parsing services which are
not needed after deployment.
Code for writing .cton files lives in the main cretonne library because it is
fairly small, and because it is useful for extracting test cases from a
deployed library.
