Add tests for fetch*() methods of Portability\Result

Extract Result from the Statement interface

Replace forward-compatible ResultStatement interfaces with Result

Report coverage unconditionally on Travis

pcov is fast enough for this to be acceptable.

Make caching layer not rely on closeCursor()

Fix final class member names according to the coding standard

Integrate with Codecov

Update doctrine/coding-standard to 8.0

Merge up mistakes

PostgreSQL100Platform inherits from PostgreSQL94Platform.

Introduce Statement::fetchFirstColumn()

Refactor portability statement into a functional composition

@link in convertException gives 404