Never update values after setting the type

Thunks are now overwritten by a helper function
`Value::finishValue(newType, payload)` (where `payload` is the
original anonymous union inside `Value`). This helps to ensure we
never update a value elsewhere, since that would be incompatible with
parallel evaluation (i.e. after a value has transitioned from being a
thunk to being a non-thunk, it should be immutable).

There were two places where this happened: `Value::mkString()` and
`ExprAttrs::eval()`.

This PR also adds a bunch of accessor functions for value contents,
like `Value::integer()` to access the integer field in the union.

src/libexpr/primops/fetchMercurial.cc

@@ -20,7 +20,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
 
     if (args[0]->type() == nAttrs) {
 
-        for (auto & attr : *args[0]->attrs) {
+        for (auto & attr : *args[0]->attrs()) {
             std::string_view n(state.symbols[attr.name]);
             if (n == "url")
                 url = state.coerceToString(attr.pos, *attr.value, context,