0.8.4

• Allow user-defined functions in symbolic expressions
• Introduced address-of and dereference operators in vector expressions
  This makes the following possible:

/*
 * Assign 42 to either y or z, depending on value of x. The trick with
 * address_of/dereference is unfortunately required because in C99 (which
 * OpenCL is based on) result of ternary operator is not an lvalue.
 */
vex::tie( *if_else( x < 0.5 ? &y : &z) ) = 42;

• vex::reduce() accepts slices of vector expressions. vex::reduce() calls may
  be nested.
• vex::element_index() optionally accepts length (number of elements). This
  allows to reduce stateless vector expressions. Could be useful e.g. for
  Monte-Carlo experiments.
• Added missing builtin functions.
• Introduced constants in vector expressions. Instances of
  std::integral_constant<T,v> or constants from vex::constants namespace (which
  are currently wrappers for boost::math::constants) will not be passed as kernel
  parameters, but will be written as literals to kernel source. Users may introduce their own constants with help of VEX_CONSTANT macro.