Handle vec4 stack version of concatenation.
Redesign support for system functions that return vec4

Redsign the handling of the return value, including a rework of
the %vpi_func syntax to carry the needed information.

Add a few more arithmetic operator instructions.
Handle vec4 part selects / vec4 cassign / repeat statements

These features need to be adapted to the vec4 stack.
vec4 cassign to part selects.
Add vec4 support for bitwise or/nor/nand/xor/xnor.
vec4 support for ufuncs and ligical AND.
Vec4 store to memories.

This also reworks the working of the %store/vec4 instruction to
take a part offset, and eliminate the %store/vec4/off instruction.
Implement vec4 basec %assign delay and event / vpiTimeVal for functions.
Add the %event instruction, remove %ix/get and %ix/get/s.
vec4 versions of a bunch of unary operators.
vec4 string literal expressions
get vpiStringVal from vec4 stack positions.