Make the get_source statement return the line numbers of top-level functions.
Implement semantic-only flag for the compiler.
Need to check for errors before tryin to access the name of the package.
Throw an exception on invalid source files.
Merge pull request #200 from mpark/semantic-only

Implement semantic-only flag for the compiler.
Return line_nums info for 'get_source' only if the source is parsed successfully.
Merge pull request #199 from mpark/line-numbers

Make the `get_source` statement return the line numbers of top-level functions