Error-proof parser

[HyeonuPark]

nal_parser is intended to be error-proof parser, means small parse errors are isolated so whole parser never fail and at last it will report positions of invalid code.

Nal's grammar provides two levels of syntactic isolation - line and block. Syntax errors can't escape containing line or block(section enclosed by bracket).

block!() parser is my attempt to do it, but I found it will totally fail in some cases, like when erroneous line includes containing block's end bracket.

To properly handle cases like this, Our parser needs to perform some bracket-matching first.

• add test cases containing erroneous lines, to check it will not spread to entire parser
• make parser error-proof