Generate ND indexing functions on demand in JIT. * src/jit-typeinfo.cc (jit_operation::~jit_operation, jit_operation::do_generate, jit_operation::generate, jit_operation::signature_cmp::operator()): New function. (jit_operation::overload): Call do_generate when lookup fails. (jit_index_operation, jit_paren_subsref, jit_paren_subsasgn): New class. (jit_typeinfo::jit_typeinfo): Update to use jit_paren_subsref and jit_paren_subsasgn. (jit_typeinfo::gen_subsref, jit_typeinfo::gen_subsasgn): Removed functions. * src/jit-typeinfo.h (jit_operation::~jit_operation, jit_operation::generate, jit_operation::do_generate): New declaration. (jit_operation::add_overload, jit_operation::overload, jit_operation::result, jit_operation::to_idx): Use signature_vec typedef. (jit_operation::singature_cmp): New class. (jit_index_operation, jit_paren_subsref, jit_paren_subsasgn): New class. (jit_typeinfo::get_scalar_ptr): Nwe function. (jit_typeinfo::gen_subsref, jit_typeinfo::gen_subsasgn): Removed declaration. * src/pt-jit.cc: New test.

## The `oregonator'.
##
## Reference:
##
##   Oscillations in chemical systems.  IV.  Limit cycle behavior in a
##   model of a real chemical reaction. Richard J. Field and Richard
##   M. Noyes, The Journal of Chemical Physics, Volume 60 Number 5,
##   March 1974.

function dx = oregonator (x, t)

  dx = zeros (3, 1);

  dx(1) = 77.27*(x(2) - x(1)*x(2) + x(1) - 8.375e-06*x(1)^2);
  dx(2) = (x(3) - x(1)*x(2) - x(2)) / 77.27;
  dx(3) = 0.161*(x(1) - x(3));

end