oceani: add some support for lvalues.

oceani: add some support for lvalues.

New linterp_exec() returns an lvalue, (pointer to value)
and individual cases can provide either an lvalue or an rvalue.

This means the RHS of assignment might not be a simple variable,
so we leave the "v = v->merged" part to the "interp exec" for Xvar.
Also, linterp_exec() might return NULL, so be careful.
This might happen, e.g., when an array index is out of bounds.

Signed-off-by: NeilBrown <neil@brown.name>