Improved compile time performance (clang/gcc)

[JessyDL]

Note the following body of work mostly affects clang and gcc. msvc will come in another PR. The performance of msvc did not noticeably change during the course of this PR.

Compile time performance was improved for clang/gcc by doing the following:

• removed the need for <fstream> in ustring.hpp by moving it to ustream.hpp
• moved psl::ecs::state_t::on_condition to its own file psl/ecs/on_condition.hpp, this removed the need for psl/ecs/details/execution.hpp which includes <execution>
• moved psl::ecs::state_t::order_by to its own file psl/ecs/order_by.hpp, this resulted in <future> no longer being needed in state.hpp
• removed dependency on ustring.hpp for assertions.hpp, and <fmt> in case of non-debug builds
• removed assertions.hpp from automatic inclusion everywhere
• moved utility::debug to a seperate file debug_utils.hpp/cpp
• moved around various parts of psl::serializer to lower compilation pressure. Future work should be done in respect of Polymorphic serialization is linked to specific encoder/decoder #70
• various other improvements to platform_utils.hpp, mostly by moving code to platform_utils.cpp

Locally this resulted in a drop of 15%-20% seconds across all TU's (measured using clang12 w/ -ftime-trace)