Parser: own the [In] grammar parameter

Parser: own the [In] grammar parameter

Previously, the [In] parameter lived in the lexer, in a byte array indexed by
bracket nesting depth.  The lexer advanced the depth while prereading, the
parser wrote at the depth it had consumed to, so a delimiter in a conditional
consequent left the write in the wrong slot and the restriction was lost:

    for (a ? (b) : c in d; false;);

The fix is to carry the parameter in njs_parser_stack_entry_t: a frame
records the value in effect when it was pushed and restores it when popped,
including on the njs_parser_reject() landing frame.  An arrow initializer now
gets [~In] as well, which it escaped before.

This also allows to remove the [In] support from the lexer. The parser
owns the parameter now. njs_lexer_t drops from 104 to 80 bytes and no longer
allocates.