Now it is possible to write anywhere in a Ligo source file
and have error messages refer to the correct file and line. This
is done by extending the lexer to recognise #line
directives (generated by CPP).
I added the stage "cpp" for the --verbose command-line option: if
given, the preprocessed source file is created in the current
directory with the name <input>.pp.li, where <input>.li is the
original input file. Also the CPP command actually run is
printed.
I added setter to modules [Pos] and [Region].
