bigint parser now returns only BigInt.

[muratgozel]

Hi,

Instead of conditionally returning BigInt (as you mention in the todo comment), the bigint type should always return BigInt so developers always know how to deal with it.

BigInt support in node.js exists as of version 10.8.0.

I haven't reviewed the codebase much. Would like to know your opinion and hope this won't break anything.

[coveralls]

Coverage remained the same at 86.573% when pulling d91586b7a66ad65a5b6142b1e289ea852b0163e5 on muratgozel:master into 91efde6dd5c9f9d482fdc3f289fbdd4222507aa2 on gajus:master.

[gajus]

This is a lot more involved change than this, so closing this PR.

Please continue discussing in one of the open issues https://github.com/gajus/slonik/issues?q=is%3Aissue+is%3Aopen+bigint