Generic codec for Map should accept tables

```hs
data Config = Config { foo :: Map Text Text } deriving Generic
```

```toml
foo = { "bar" = "baz" }
```

```
tomland decode error:  The following fields are present in TOML but not used:
[foo]
  "bar" = "baz"
```

There is no reason for this to fail, the intent is clear.