core: added scanning for results

[brennanjl]

Sorry, I should've split this up into separate PRs. It does 3 things:

1. It adds the ability to scan into values when using the Go client.
2. It moves the decimal package into types. It also renames some of the exported functions related to decimals to make it clear that they are for decimals (e.g. NewExplicit -> NewDecimalExplicit). Previously, it was clear that these were for decimals because they were in the decimals package, but now that is not the case.
3. It renames references to the Kuneiform's decimal type to numeric, to make it match Postgres more closely. This was actually a change I made in the new engine a while ago., there were just a few straggling things I hadn't updated. decimal still works as an alias to numeric.