Support more forms of assignment in constant expressions

[udoprog]

This adds support for targeting in constant expression, i.e. the following kind of expressions:

let object = #{};
object.test = #{};
object.test.test = 42;

However, these currently do not have the same semantics as regular Rune execution. Non-primitive rhs values are cloned, rather than refcounted. To change this, we need to introduce the use of a Value-like structure in the IrInterpreter which then converts the computed result into ConstValue once it has been evaluated.