symbols: add LevelInfo

symbols: add LevelInfo

Add a LevelInfo to hold a single array of level specific info inside
a GroupInfo, instead of keeping the acts, symsMapIndex and
symsMapNumEntries arrays and the numLevels field separate and in sync.
This simplifies the code, and goes a long way toward making  the
key-merging code somewhat understandable. Also uses less memory.

Note that the syms array is still in GroupInfo for now, with the levels
holding offsets into it.

Signed-off-by: Ran Benita <ran234@gmail.com>