Updated Transformation::evaluate signature to allow for in-place updates, removing unnecessary heap allocated copies.

- Renamed Transformation::evaluate to Transformation::transform to avoid
  confusion with Action's overload methods.
- Updated Transformation::transform signature to receive the value by
  reference and perform the transformation inline, if possible.
  - Some transformations still need to use a temporary std::string to
    perform their work, and then copy the result back.
- Made Transformation::transform methods const and updated Transaction
  parameter to be const.
  - Transaction parameter could not be removed because it's used by just
    a single transformation, UrlDecodeUni.
- Removed std::string Action::evaluate(const std::string &exp,
  Transaction *transaction); which was only implemented by
  Transformation but was not used from the base class, but only after
  downcasting to Transformation, so it can just be declared there (and
  not pollute other actions with a default member implementation -that
  does nothing- which is never called).

test/coding_style_suppressions.txt

@@ -13,7 +13,6 @@
 ./src/utils/acmp.cc
 ./src/utils/acmp.h
 ./src/utils/mbedtls/
-./src/utils/md5.cc
 ./src/utils/md5.h
 ./src/utils/msc_tree.cc
 ./src/utils/msc_tree.h