Remove global variable meta

[zhanghongce]

• Remove global variable meta which is unused
• Store range info in ast_port_declaration
• Copy as_bits field to avoid being changed

List of changes

• src/verilog_ast.h: remove meta from line 1092
• src/verilog_parser.y : use range_o to replace NULL for non-terminal: port_declaration_l
• src/verilog_ast.c: use ast_strdup in function ast_new_number

Fixes:

• Now it is fine for others to include the headers in multiple source files.
• ast_number->as_bits are usable
• Range can be obtained in ast_port_declaration (for new style module declaration)