Cleanup and improve stack map stream

- transform AddStackMapEntry into BeginStackMapEntry/EndStackMapEntry.
This allows for nicer code and less assumptions when searching for equal
dex register maps.
- store the components sizes and their start positions as fields to
avoid re-computation.
- store the current stack map entry as a field to avoid the copy
semantic when updating its value in the stack maps array.
- remove redundant methods and fix visibility for the remaining ones.

Change-Id: Ica2d2969d7e15993bdbf8bc41d9df083cddafd24