Teach abs how to handle longs

this let's us import random

src/runtime/builtin_modules/builtins.cpp

@@ -90,6 +90,8 @@ extern "C" Box* abs_(Box* x) {
     } else if (x->cls == float_cls) {
         double d = static_cast<BoxedFloat*>(x)->d;
         return boxFloat(d >= 0 ? d : -d);
+    } else if (x->cls == long_cls) {
+        return longAbs(static_cast<BoxedLong*>(x));
     } else {
         RELEASE_ASSERT(0, "%s", getTypeName(x)->c_str());
     }

src/runtime/long.cpp

@@ -224,6 +224,14 @@ Box* longNeg(BoxedLong* v1) {
     return r;
 }
 
+Box* longAbs(BoxedLong* v1) {
+    assert(isSubclass(v1->cls, long_cls));
+    BoxedLong* r = new BoxedLong(long_cls);
+    mpz_init(r->n);
+    mpz_abs(r->n, v1->n);
+    return r;
+}
+
 Box* longAdd(BoxedLong* v1, Box* _v2) {
     if (!isSubclass(v1->cls, long_cls))
         raiseExcHelper(TypeError, "descriptor '__add__' requires a 'long' object but received a '%s'",

src/runtime/long.h

@@ -37,6 +37,7 @@ extern "C" Box* createLong(const std::string* s);
 extern "C" BoxedLong* boxLong(int64_t n);
 
 Box* longNeg(BoxedLong* lhs);
+Box* longAbs(BoxedLong* v1);
 
 Box* longAdd(BoxedLong* lhs, Box* rhs);
 Box* longSub(BoxedLong* lhs, Box* rhs);