Type-Parameterize Core.Box

[uniment]

There are performance optimizations that do not occur when Box accesses are typeasserted, which instead require the Box to be type-parameterized (example). The first step will be to parameterize Core.Box, and then other improvements can be made.

By this edit, type parameterization must be performed explicitly; otherwise behavior is the same as before (e.g. Box(1) is a Box{Any}(1); call Box{Int}(1) to construct a Box{Int}).