Added proper parsing of symbols
Got rid of repr module
removed `id` and private markers
test/scribble/core_test: fixed test group names
test/scribble/core_test: added a testcase for @@
test/scribble/core_test: added a separate testcase for nil recognition
src/scribble/symbol: wrapping symbols in a type in order to process nils correctly
src/scribble/reader: returning reader, not nil if nothing is read; and checking for equivalence to reader properly