Core: Move math logic to headers where applicable

[Repiteo]

This PR aims to migrate applicable math code from source to headers where applicable. That is:

• Relevant headers are already present
• Not internal functions that should be obfuscated
• Functions that would obviously and immediately benefit from inlining

This doubles as a means of guaging header bloat in a constrained environment, and deciding which headers shouldn't be included by default. The most obvious example is the removal of ustring.h includes from several headers, causing a handful of functions to achieve the inverse effect & migrate to their compilation file instead

Goals:

• Migrate relevant functions from compilation files to header files.
• Ensuring functions are explicitly inlined through our inlining macros.
• Reapply inlining macros to constexpr functions during above pass.
• Integrate constexpr to functions where effortless to do so, keeping with the inlining trend.
• Trim #include headers to the minimum necessary to function.