Introduce Context wrapper for user-defined functions.
Introduce Context wrapper for user-defined functions.

This commit adds get/set auxilliary data for arguments; more to come.

jgallagher/rusqlite,jgallagher/rusqlite,mikelodder7/rusqlcipher,mikelodder7/rusqlcipher,xinghun92/rusqlite,mikelodder7/rusqlcipher,gwenn/rusqlite,jgallagher/rusqlite,gwenn/rusqlite,gwenn/rusqlite,xinghun92/rusqlite,xinghun92/rusqlite