math: initial implementation of the general math::truncate_degree() function, including the related type trait.
t_subs_series: assert that the template param Derived follows the CRTP.
piranha.hpp: add todo item.
power_series: make it a CRTP class, in preparation for the truncation work.
Replace a few usages of symbol_set::positions with const counterparts - there is no reason these objects should be mutable.
type_traits: add a simple true type-trait in the detail namespace, for use with decltype() SFINAE.
power_series: initial implementation of the degree truncation methods, testing and docs to be finished.
power_series: more truncation testing.
power_series: for truncation, change an implementation detail, documentation and more test bits.
power_series: add a performance test.
pyranha: initial exposition and testing of the degree truncation method.
pyranha: fix doc typos.
pyranha: bit of internal documentation.
pyranha: fix another doc typo.
pyranha: for the moment expose only truncation wrt piranha::integer.

That is the only exponent type at the moment.
pyranha: doc and doctest for the truncate_degree() function.
pyranha: some small cosmetic work on the sphinx docs.
pyranha: some more minor doc bits.
pyranha: minor bit of internal doc.
power_series: some minor test additions.