Merge #146

146: Implement the 'as32Bit[Un]signedValue' primitives. r=vext01 a=ltratt

Java SOM crashes on big integers, so I have defined what I believe to be appropriate semantics for them which appears to be not to perform sign extension in the conversion. For example:

```
  ((1 << 60) - 1) as32BitUnsignedValue
```

in Java SOM returns 4294967295, which is bigger than a signed 32 bit can represent. This commit preserves that behaviour.

Co-authored-by: Laurence Tratt <[email protected]>

lang_tests/to32bits.som

@@ -0,0 +1,43 @@
+"
+VM:
+  status: success
+  stdout:
+    0
+    1
+    4294967295
+    4294967295
+    0
+    4294967295
+    0
+    4294967295
+    0
+    1
+    -1
+    -1
+    0
+    -1
+    0
+    -1
+"
+
+to32bits = (
+    run = (
+        0 as32BitUnsignedValue println.
+        1 as32BitUnsignedValue println.
+        -1 as32BitUnsignedValue println.
+        4294967295 as32BitUnsignedValue println.
+        (1 << 60) as32BitUnsignedValue println.
+        ((1 << 60) - 1) as32BitUnsignedValue println.
+        (1 << 200) as32BitUnsignedValue println.
+        ((1 << 200) - 1) as32BitUnsignedValue println.
+
+        0 as32BitSignedValue println.
+        1 as32BitSignedValue println.
+        -1 as32BitSignedValue println.
+        4294967295 as32BitSignedValue println.
+        (1 << 60) as32BitSignedValue println.
+        ((1 << 60) - 1) as32BitSignedValue println.
+        (1 << 200) as32BitSignedValue println.
+        ((1 << 200) - 1) as32BitSignedValue println.
+    )
+)

src/lib/vm/core.rs

@@ -636,8 +636,34 @@ impl VM {
                 self.stack.push(v);
                 SendReturn::Val
             }
-            Primitive::As32BitSignedValue => todo!(),
-            Primitive::As32BitUnsignedValue => todo!(),
+            Primitive::As32BitSignedValue => {
+                let i = if let Some(i) = rcv.as_isize(self) {
+                    i as i32 as isize
+                } else {
+                    rcv.downcast::<ArbInt>(self)
+                        .unwrap()
+                        .bigint()
+                        .to_u32_digits()
+                        .1[0] as i32 as isize
+                };
+                let v = Val::from_isize(self, i as isize);
+                self.stack.push(v);
+                SendReturn::Val
+            }
+            Primitive::As32BitUnsignedValue => {
+                let i = if let Some(i) = rcv.as_isize(self) {
+                    i as u32
+                } else {
+                    rcv.downcast::<ArbInt>(self)
+                        .unwrap()
+                        .bigint()
+                        .to_u32_digits()
+                        .1[0] as u32
+                };
+                let v = Val::from_isize(self, i as isize);
+                self.stack.push(v);
+                SendReturn::Val
+            }
             Primitive::At => {
                 let rcv_tobj = stry!(rcv.tobj(self));
                 let arr = stry!(rcv_tobj.to_array());