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>
diff --git a/src/sksl/SkSLByteCodeGenerator.cpp b/src/sksl/SkSLByteCodeGenerator.cpp
index e07a22e..acfbac5 100644
--- a/src/sksl/SkSLByteCodeGenerator.cpp
+++ b/src/sksl/SkSLByteCodeGenerator.cpp
@@ -73,7 +73,7 @@
 //                    SkASSERT(!(declVar->fModifiers.fFlags & Modifiers::kIn_Flag));
                     if (declVar->fModifiers.fFlags & Modifiers::kUniform_Flag) {
                         for (int i = SlotCount(declVar->fType); i > 0; --i) {
-                            fOutput->fInputSlots.push_back(fOutput->fGlobalCount++);
+                            fOutput->fUniformSlots.push_back(fOutput->fGlobalCount++);
                         }
                     } else {
                         fOutput->fGlobalCount += SlotCount(declVar->fType);