Support lifted coercions of file schemas into compatible read schemas.

Pig and Hive assume certain values are nullable but should support
reading from Parquet files that do not have a representation of null
values. This patch introduces the notion type lifting. Lifted values
exist at runtime merely to provide a translation between the disk schema
and requested schema.

For example, a Parquet file with the following schema can not be read
from Pig or Hive:

```
message file_schema {
  optional binary key (UTF8);
  repeated group records {
    optional binary name (UTF8);
    optional binary type;
  }
}
```

The requested schema that e.g. Pig requires wraps the `records` group
in an optional group in order to provide an explicit representation
of a null value.

```
message requested_schema {
  optional binary key (UTF8);
  optional group records (LIST) {
    repeated group records {
      optional binary name (UTF8);
      optional binary type;
    }
  }
}
```

The file and requested schemas are not equal but are compatible for
reads with a simple transformation. When a nullable field present
in the requested schema but not on the file schema a GroupType wrapper
is inserted. This lifts non-nullable values into a nullable value.