Use RCP<const A>

This was done using:

find . -name \*.cpp | xargs sed -i 's/RCP</RCP<const\ /g'
find . -name \*.h | xargs sed -i 's/RCP</RCP<const\ /g'
Manual fixes for const

These had to be changed by hand.
Update benchmarks
Update the Python wrappers
Implement Pow.get_base() and get_exp()
Fix a hack in Python wrappers
Merge pull request #114 from certik/const

Make the code const correct