refactor(numerics): rename power operator to pow, the idea is to follow the C/C++ name convention for the power operator

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(interval): organize template source code using the approach described at http://www.codeproject.com/Articles/3515/How-To-Organize-Template-Source-Code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(interval): move interval unit tests to tests/util/interval

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>