runtime: Don't call regular method if assignment hash matches

This is a pretty nasty breakage in lookup code that I somehow didn't
catch in tests, so I'm adding one for it now. This caused all unary
methods that had a single-keyword variant to ignore the single-keyword
variant and only use the unary one. This also broke argument sentinel
checks but I never noticed it. Ouch!

Author: sin-ack

src/runtime/objects/slots.zig

@@ -145,7 +145,7 @@ pub fn slotsLookup(
 
         const matches_constant = selector.equals(slot_selector);
         const matches_assignable = selector.canAssignTo(slot_selector);
-        if (matches_constant or matches_assignable) {
+        if (matches_constant or (matches_assignable and slot.isAssignable())) {
             return .{
                 .Found = .{
                     .object = @ptrCast(object),

tests/interpreter/dontCallRegularMethodIfAssignmentLikeMessageIsSent.self

@@ -0,0 +1,18 @@
+"Checks for a regression in the lookup code. Suppose we had this object:
+(|
+    foo = 1.
+    foo: x = (""Do something."").
+|)
+We would accidentally send a message to `foo' when sending `foo:' to this
+object."
+
+(|
+    parent* = std traits singleton.
+
+    foo = (false assert: 'Lookup code broke!').
+    foo: x = ("Hooray, no regression!").
+
+    run = (
+        foo: 'x'.
+    ).
+|) run.