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()

Integrate with Codecov

Update doctrine/coding-standard to 8.0

Introduce Statement::fetchFirstColumn()

@link in convertException gives 404

The links in convertException gives 404, i'm not 100% if I updated them correct, or if the e.g. should be update to 8.0 already.

https://dev.mysql.com/doc/refman/8.0/en/server-error-reference.html
https://dev.mysql.com/doc/refman/8.0/en/client-error-reference.html

Additional changes based on the discussion in #4007

This will allow for subsequent fetches in different modes.

Psalm 6

The php docs are not very clear about that one.

It confuses both Psalm and PHPStan, and involves complicated mocking.

This return type is conditional to $params['wrapperClass']. Luckily,
recent versions of Psalm allow documenting it properly.
See https://github.com/vimeo/psalm/issues/3277

Note that phpstan is not able to understand this yet, but still attempts
to, hence the extra ignore rules.

This hack is temporary and should be removed on 3.0.x