Halide intrinsics need a central documentation location

[steven-johnson]

Halide has a number of intrinsics that require proper documentation (ie their use is not obvious from casual inspection); there has never been an agreed-upon place for said documentation, and so that does exist is typically in comments on a CodeGen class. We should fix this, e.g. by moving all Intrinsics declaration to a unique header file, and require the documentation there.