strict: generic cast functions for conversion to strict types

This package contains generic cast functions enabling providers to convert the values obtained from source databases to the strictly-typed values expected in heterogenous transfers. The main purpose of these functions is to just call the cast, providing a single-line "interface" to do that in concrete providers' type switch statements.

The functions in this package are divided into two categories: Expected and Unexpected. They work the same way in regards to the type / value conversion. They differ in just one way. The Expected functions return errors different from the errors returned by the Unexpected functions when a cast error occurs. The difference depends on the type of the incoming value being casted.