Context.Rem fails for Rem(100E100, 10)

I attempted to run the following test:

```golang
func TestRem(t *testing.T) {
a := apd.New(100, 100)
b := apd.New(10, 1)
var rem apd.Decimal
_, err := apd.BaseContext.Rem(&rem, a, b)
if err != nil {
t.Fatal(err)
}

if !rem.IsZero() {
t.Fatal("expected zero")
}
}
```

I would expect this test to pass, but it fails:

```
=== RUN TestRem
rem_test.go:72: division impossible
--- FAIL: TestRem (0.00s)

FAIL
```

apd.BaseContext.Rem returns `division impossible`, which (I believe) is an incorrect result in this case.

Start with Context.Rem and reproduce the failing case from the issue's TestRem example, using rem_test.go around line 72 as the test location mentioned by the failure. Trace why Rem(100E100, 10) returns division impossible; done means the case completes without an error and produces a zero remainder.

Written by the indexing model from the issue text.