LHS of assignment is no longer limited to one pair of subscript
brackets. It can be chained with dot and subscript freely like RHS
variables. This allows direct assignment into the list that is an
element in an array of list. Related unit tests are added and passed.

Need work on error handling. There are many system or user errors are
not handled properly. They are catched in the main program but the
handler just simply reissue them (not very helpful).
Improve AST error handling by add common error catch code in all eval
method. The common error catch code is included to the method using the
@ directive. This helps to show meaningful error message by always
reference to the input string. Error handling of parser and lexer is
also centralized.
NullNode created from {} correctly shows the lexeme.
Implicit integers and unsigned integers are now auto-promoted to their
corresponding long or long64 types when necessary.
The main midle routine now always return !NULL if anything goes wrong
during execution instead of message out sometimes. The error output
keyword will always contain the error message.