Update README.md
Refactor any to not use any const_casts

Consider this a followup to #115. There's no need to use const_cast,
since it's very easy to make it work in a const-correct manner.
Merge pull request #137 from SpecLad/any-const

Refactor any to not use any const_casts
Merge commit '5590307f4f16f74dd1310513f808824c4c18dbb1'
Fixed abstract errors.