Change ByteCode and ByteCodeFunction to classes

The lack of encapsulation was finally starting to bother me. Had to
change the Interpreter namespace to a struct so that it could be
friended, but otherwise this was a nice and simple cleanup.

Also updated the comments on the two run functions, and renamed
fInputSlots to fUniformSlots, to reflect recent clarification
around in vs. uniform.

Change-Id: I24bbc59778b3ab6448bffcf98133d5c149a060a9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244883
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed