Replace `LaneIdx` immediates with integers

[penzn]

Change current extract and replace lane operations to use immediate indices within lower 128 bits of the vector (compatible to current standard). Keep description of immediates identical to what is used in current SIMD.

Add two more flavors for extract and replace lane operations: one containing a bounds check and the other taking the index value modulo number of lanes in the vector.

Update opcodes to reflect new operations and changes in the names.

Minor housekeeping: set length should take an immediate, note on AVX512.

[penzn]

In short, there are three approaches to lane indices in this set up:

• Limit immediates to what values that are always valid (for fallback to scalar this would just one, but we can fallback on simd128)
• Check that index is within bounds at runtime
• Take index module the number of lanes

And we can have all of that, especially at this stage of the proposal.