expression eval: Fix EEOP_JSON_CONSTRUCTOR and EEOP_JSONEXPR size.

expression eval: Fix EEOP_JSON_CONSTRUCTOR and EEOP_JSONEXPR size.

The new expression step types increased the size of ExprEvalStep by ~4 for all
types of expression steps, slowing down expression evaluation noticeably. Move
them out of line.

There's other issues with these expression steps, but addressing them is
largely independent of this aspect.

Author: Andres Freund <[email protected]>
Reviewed-By: Andrew Dunstan <[email protected]>
Discussion: https://postgr.es/m/20220616233130[email protected]
Backpatch: 15-