Add reader state inspection

Add reader state (Reader.State class) to inspect the progress and internal
states of the reader.
Modify the constructor overloads of ParseException class.
Add reader context handlings

Modify the rules in built-in grammars to handle reader the context stack.
Add transiting expression rules for the pattern grammar.
Fix the transiting expression rule to the standard grammar in the alternative
grammar (re-implement as an extended term).
Improve error outputs of YacqRepl to use information from the reader context.