We now call Preprocessor::process_and_lex() and pass the result to the
parser.
Doing the lexing in the preprocessor will allow us to maintain the
original position information of tokens after substituting definitions.
Previously, ASTNode::dump() used outln() for output, which meant it
always wrote its output to stdout.
After this commit, ASTNode::dump() receives an 'output' argument (which
is stdout by default). This enables writing the output to somewhere
else.
This will be useful for testing the LibCpp Parser with the output of
ASTNode::dump.
