explicit resource management: added 'using' variable declarations (initial patch...

explicit resource management: added 'using' variable declarations (initial patch by bnoordhuis and saghul) - simplified the code by removing 'OP_dispose_scope' as the disposal code can be generated on the fly - avoid declaring 'using' as a keyword - use peek_token() to efficiently test the presence of a 'using' declaration - added DECL_MASK_USING to identify the statements where 'using' declarations are allowed